Evolutionary Anthropology 81

What Can Dolphins Tell Us About Primate Evolution? LORI MARINO

Fifty-five million years ago, a furry, hoofed mammal about the size of a dog ven- tured into the shallow brackish remnant of the Tethys Sea and set its descendants on a path that would lead to their complete abandonment of the land. These early ancestors of cetaceans (dolphins, porpoises, and whales) thereafter set on an evo- lutionary course that is arguably the most unusual of any mammal that ever lived. Primates and cetaceans, because of their adaptation to exclusively different physi- cal environments, have had essentially nothing to do with each other throughout their evolution as distinct orders. In fact, the closest phylogenetic relatives of ceta- ceans are even-toed ungulates.

Given this independence, what pos- sible relevance could the study of ce- taceans have for understanding primate behavioral evolution? And what could we possibly learn about primates by comparing them to ceta- ceans that we could not learn from studying primates themselves? The answer to these questions is straight- forward By comparing cetaceans and primates we can differentiate between the characteristics that are unique to primates and those that result from general factors that have shaped be- havioral evolution in mammals. These two groups’ disparate evolutionary histories and consequent organiza- tional differences stand in stark con- trast to their shared level of

Lori Marino is an Assistant Professor of Biology and Anthropology at Emory University and Research Associate at the Yerkes Regional Primate Research Center, both in Atlanta. For the past few years she has studied brain-behavior relations in primates and cetaceans. She received her doctorate in biopsychology from the State University of New York at Albany in 1995. She is the author of several papers and book chapters on comparative brain-behavior relations in cetaceans and primates. Currently, she is examining behavioral laterality and its relation to brain hemispheric asymmetry in both chimpanzees and bottlenose dolphins.

Key words: cetaceans, encephalization quotients, brain size, brain evolution

neurobiological advancement. The co- existence of these similarities and dif- ferences form the basis for a novel comparative approach to under- standing the nature of primate and ce- tacean behavioral evolution. With some notable exceptions,1-5 few re- searchers have attempted compara- tive analyses of primates and cetaceans on either an experimental or behavioral ecological level. More specifically, there were, until recently, scarcely any direct, formal compara- tive analyses of brain-behavior rela- tions between primates and cetaceans. What follows are examples of what has been learned about pri- mates from cetaceans through recent direct comparisons of brain size and behavior.

HOMlNlD ENCEPHALIZATION IN PERSPECTIVE

Enlargement of the brain is consid- ered to be one of the hallmarks of pri- mate, and especially hominid, evolution. The encephalization quo- tient (EQ) is a measure of relative brain size that takes into account al- lometric trends in brain and body pro- portions. Primate EQs are generally higher than average in comparison to those of most mammals.6 Further- more, the hominid lineage is noted for its steep increase in relative brain size, which has given rise to humans, the most highly encephalized mammal

existing to date. Clearly, our perspec- tive on the uniqueness of hominid encephalization and our under- standing of its causes can be enhanced by comparing hominid trends with those in cetaceans.

The following discussion is based on a series of comparative studies7fa in which EQs for 60 anthropoid primate species (monkeys, apes, and humans) and 21 odontocete species (toothed whales, dolphins, and porpoises) were calculated from a large base of brain and body-weight data. Two forms of EQ are discussed below: Jerison’s9 EQ.67, representing the degree of encephalization of each species in the sample in comparison to a large sam- ple of other mammals, and EQ.72, which was empirically derived’ and represents the degree of encephaliza- tion in each species in comparison to each of the other species in the sam- ple.

Figure 1 offers a broad view of the relative levels of encephalization among each of the living anthropoid primate and odontocete families as members of a general mammalian group (i.e., Jerison’s EQ.67). Modern humans, with an EQ.67 value of 7.06, are the most encephalized. However, the next highest set of values is not that of the closest living phylogenetic relatives to humans, the great apes (with a mean EQ.67 of 1.91), but that of the delphinidae (dolphins and toothed whales), with a mean EQ.67 of 3.78. The delphinidae, in turn, are followed by phocoenidae (porpoises), with a mean EQ.67 of 3.07. In fact, the delphinidae, which have a signifi- cantly higher EQ.67 than other odon- tocete families except the porpoises, have a significantly higher EQ.67 level than do any nonhuman anthropoid primate families, including the great apes.

This pattern is maintained even

82 Evolutionary Anthropology ARTICLES

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EQ.67 Figure 1. Mean EQ.67 for each anthropoid primate and odontocete family in the present sample. The delphinidae (with a mean EQ.67 of 3.78) have a significantly higher EQ level than that of any primate and cetacean group except Homo sapiens and the phocoenidae (porpoises).

when the effect of disproportionately large body size in males of the two most highly sexually dimorphic great ape species, orangutans and gorillas, is taken into account. That is, delphini- dae have a higher EQ.67 than do fe- male orangutans and gorillas, even though the encephalization level of fe- males in these two great ape species is not subject to the allometric affects of large body size as it is in the males of these species.

These findings are primarily due to a subset of four dolphin species that have the highest encephalization level (mean EQ.67 = 4.3) of all modern mammals except humans. These spe- cies are the tucuxi dolphin, the Pacific white-sided dolphin, the common dol- phin, and the bottlenose dolphin. When these four species are compared with modem humans, using the pre- sent anthropoid primate-odontocete sample as the reference group (ie., EQ.72), the differences between the levels of encephalization of modem humans and these four dolphin spe-

cies as members of the same group are diminished. Figure 2 shows that the

five top-ranked species among anthro- poid primates and odontocetes are our own (EQ.72 = 2.88), the tucuxi (EQ.72 = 1.89), the Pacific white-sided dolphin (EQ.72 = 1.82), the common dolphin (EQ.72 = 1.74), and the bot- tlenose dolphin (EQ.72 = 1.58). Al- though several primate and odontocete species come close to the range of EQ.72 levels achieved by these dolphin species, once again the closest relative to humans, the com- mon chimpanzee, is not nearly as encephalized (EQ.72 of 0.97) as the dolphin species are.

The purpose here is not to focus on the fact that the great apes have lower encephalization levels than do hu- mans and dolphins. Rather, the objec- tive is to point out two important implications of these findings regard- ing how human encephalization and intelligence are viewed. First, the pres- ence of these highly encephalized dol- phin species provides evidence of a substantial narrowing of the gap be- tween human and nonhuman levels of encephalization. Second, the presence of nonprimate EQs that are much closer to that of humans than the EQs of any other primate indicates that similar levels of encephalization can emerge in very different phylogenetic lineages.

In a further effort to place these re-

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EQ.72 Figure 2. Mean EQ.72 forthe five top-ranking species in the present primate and odontocetesample and the great apes. Four of these are dolphins (white-sided dolphin, Tucuxi dolphin, bottlenose dolphin, and common dolphin).

ARTICLES Evolutionary Anthropology 83

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Colnmoo dolphin Bottlenose dolphin

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Figure 3. Approximate 13.67 for each of the four major hominid ancestral and modern species at the time of their emergence, along with the EQ.67 values for four modem delphinids and one modern porpoise. It was not until 1 mya that Homo exceeded dolphins in encephalization level, and even then not substantially.

sults in perspective, EQ.67 values were obtained for ancestral hominid line- ages9-10 and compared with five of the most highly encephalized modern odontocete species. Figure 3 shows cranial volume-based EQ.67 values for each of the four major hominid ances- tral and modern groups (Australopi- thecus afarensis, Homo habilis, H . erectus, and modern H. supiem) at the approximate time of their emergence, along with the EQ.67 values for the four dolphin species discussed earlier and one modern porpoise species (Dall's porpoise).

As shown in Figure 3, these five odontocete species have attained an encephalization level well above that of the first hominids (e.g., A. afarensis), and with the exception of D a b por- poise, equal to or above the first mem- ber of our own genus, H. habilis (EQ = 4 .26) . Furthermore, the level of encephalization achieved by H, erectus (EQ = 5.34), the immediate precursor to H. sapiens, was not substantially greater than that of the tucuxi (EQ = 4.56) and the Pacific white-sided dol- phin (EQ = 4.55).

In contrast to the relatively recent emergence of a human level of encephalization, fossil evidence on the cranial capacity of cetaceans indicates that many of the odontocetes (as well as mysticetes) had attained their pre- sent level of encephalization approxi- mately 15 mya?JI Thus, as recently as 2.0 mya, the most highly encephalized mammals to have evolved were not

hominids but dolphins. This puts into perspective arguments about the uniqueness or improbability of homi- nid levels of encepha1ization.l2 Sec- ond, the fact that dolphins and other

... as recently as 2.0 mya, the most highly encephalized mammals to have evolved were not hominids but dolphins.

cetaceans have been in stasis for much longer than has been the case in the hominid line offers a singular oppor- tunity to understand how brains with encephalization levels that are rela- tively close to those of humans are maintained over long periods. The odontocete data suggest that in- creased encephalization is not neces- sarily autocatalytic and can level off after a large spurt.

Although cetaceans have possessed their big brains for much longer than humans have theirs, there is a tempo- ral aspect of cetacean and hominid brain evolution that is intriguingly similar. Hominid evolution is charac- terized by an extremely steep increase in relative brain size over a short time. Indeed, hominid relative brain size tri-

pled in two million years.IOIt has been suggested that this rapid spurt in encephalization is unique among mammals and stands in contrast to the slower-paced general enlargement of brains over the course of vertebrate evolution. Furthermore , various authors have suggested that qualita- tively different complex charac- teristics may emerge from rapid evolutionary ~ h a n g e . ~ . ' ~ Therefore, important implications are associated with the question of whether or not the rapidity and magnitude of hominid encephalization is unique among mammals.

Cetacean brain evolution offers an- other example of a rapid increase in relative brain size in addition to homi- nid brain evolution. The period in ce- tacean brain evolution that is analogous to the last few million years of human brain evolution from the perspective of increasing encephaliza- tion is between approximately 25 and 38 million years ago, from the early Oligocene to the early M i ~ c e n e . ~ ~ " Fossil evidence suggests that some time before the early Oligocene, the first suborder of cetaceans, the ar- chaeocetes, became extinct.lS,l6 At the same time, early modern forms of odontocetes emerged, and had brains of substantially larger relative sizes than did the a rchaeoce te~ .~?~~ More specifically, within approximately 5 to 10 million years there was, according to Jerison's9 measurements, a three- fold augmentation in EQ, from about 1.5 for late archaeocetes to a range of 3.5 to 4.5 for some of the early dolphin f o r m ~ . ~ J ~ Moreover, there is evidence that the demise of the archaeocetes and the emergence of early modern odontocete forms with higher encephalization levels occurred at a time of major decreases in southern oceanic temperatures and dramatic biotic turnover.I5 Because many theo- ries about hominidization revolve around climatic changes," the ceta- cean evidence provides another po- tential parallel to hominid evolution and lends some support to the im- portance of dramatic selective shifts in brain evolution. Further study of both cetacean and hominid brain evolution against the backdrop of time and physical environmental change may be usehl for exploring hypothe-

84 Evolutionary Anthropology ARTICLES

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Neocortex ratio (to rest of brain) Figure 4A. The relation between maximum pod size and EQ.67 in odontocetes. Three examples are shown. Left to right: pygmy sperm whale, Doll‘s porpoise. and Pacific whitesided dolphin. 4B. The relationship between the neocortex/whole brain ratio and mean group size in primates. Redrawn from Dunb~r .*~

ses about what kinds of selective pres- sures could have led to the kind of rapid increase in brain size that char- acterized our own lineage.

COMPARATIVE BRAIN BEHAVIOR RELATIONS: A FRESH APPROACH

TO PRIMATE BEHAVIORAL EVOLUTION

One of the most important ways that cetacean brain evolution can shed

light on primate brain evolution is by providing a comparative model of the relative importance of brain size and brain organization in the evolution of intelligence. There are striking differ- ences in the morphological organiza- tion and cytoarchitectural composition of cetacean and primate brains. These differences, which are evidence of highly different independent evolu- tionary histories, are apparent at the

level of overall shape and cortical or- ganization down to basic cortical cy- toarchitecture.

Cetacean b r a i n ~ * ~ J ~ and primate brainsz0 are characterized by different “cutting edge” features. The cetacean brain evinces a proliferation of tissue along the lateral axis in the “temporal” and “parietal” regions. The leading trend in primate brain evolution has been toward increased frontal and, to some extent, occipital augmentation. Also, cetacean brains are augmented along epihippocampal regions, whereas primate brains are well devel- oped along epistriatal regions. Fur- thermore, whereas the primate brain contains three contiguous lobes: the rhinic, limbic, and supralimbic, the

... the relation between encephalization and social complexity is not unique to primates.

cetacean brain is organized around three distinct concentric tiers of tissue that include not only the limbic and supralimbic regions, but also an en- tirely unique paralimbic region, the function of which is largely unknown. The segregation of the limbic and su- pralimbic regions by an interposed paralimbic lobe is a radical departure from the typical terrestrial mammal- ian pattern of cortical evolution. The cortical topography of functional pro- jection regions in the cetacean brain also stands in contrast to that of pri- mates, in which the projection zones are interposed by extensive nonpro- jection regions. In cetaceans, the ma- jor projection zones exhibit a pattern of cortical adjacency that is unique among mammalian brains of this size.

The cytoarchitectures of the ceta- cean and primate cortex also differ markedly Primate brains are charac- terized by six well-differentiated lay- ers in which the cells are of diverse types and sizes2’ On the other hand, the cetacean cortex is composed of five less distinct laminae consisting of

ARTICLES Evolutionary Anthropology 85

a more homogeneous range of cell types. The high degree of granulariza- tion, which is so prominent a feature in primate brains, is absent in cetace- ans because of the predominance of extremely large pyramidal cells with long axons that are present over stel- late cells of the type found in primate brains.

The co-occurrence of these vast dif- ferences in organization and closeness in encephalization level between hu- mans and dolphins also offers the op- portunity to test specific hypotheses about the relationship between brain and behavior. It has been postulated, for example, that some form of social complexity is associated with higher intelligence in primatesJ2 Further- more, recent studies have shown a positive correlation between meas- ures related to encephalization and so- cial group size in primates.23.24 Likewise, in many dolphin species there is a significant positive correla- tion between EQ and pod size (see Fig. 4).7 Within cetaceans there are many nested levels of social group- ing, but the pod appears to be the social unit most parallel to the level of grouping examined in primates studies of this kind. Therefore, these findings suggest that the relation- ship between relative brain size and one measure of social complexity, group size, is similar in cetaceans and primates.

These findings also tentatively pro- vide support for the hypothesis that the relation between encephalization and social complexity is not unique to primates. Studies of the relation be- tween encephalization and other life- history variables have revealed other similarities and differences between odontocetes and primates. For in- stance, the relationship of encephali- zation to gestation length and body weight does not follow the same pat-

tern among odontocetes that it does in primates and most other mammals.* However, the relation between diving patterns and encephalization in odon- tocetes does conform to hypotheses about oxygenation demands based on primate and other terrestrial mam- malian studies.25 Therefore, although it is a new approach, the comparison of brain-behavior relations between primates and cetaceans is being re- vealed as a productive way of examin- ing hypotheses about brain and behavioral evolution.

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11 Jerison HJ (1 978) Brain and intelligence in whales. In Frost S (ed), Whales and Whaling, vol. 2 , pp 159-197. Canberra: C.J. Thompson. 12 Mayr E (1994) Does it pay to acquire high intelligence? Perspectives in biology and medicine, pp. 150-1 54. 13 Calvin WH (1988) Fast tracks to intelli- gence (considerations from neurobiology and evolutionary biology). In M a n G (ed), Bioas- tronomy. The Next Steps, pp 237-246. Dor- drecht: Kluwer Academic Publishers. 14 Gould SJ (1991) Exaptation: A crucial tool for an evolutionary psychology. J Soc Issues 47:43-65. 15 Fordyce RE (1980) Whale evolution and Oligocene southern ocean environments. Pa- leogeogr, Paleoclimatol, Paleoecol 3/:319- 336. 16 Gaskin DE (1982) The Ecology of‘ Whales and Dolphins. London: Heinemann Educa- tional Book, Ltd. 1 7 Vrba ES (1988) Late Pliocene climatic events and hominid evolution. In Grine F (ed), The Evolutionary History ofthe Robust Austra- lopithecines, pp 405-426. New York: Aldine. 18 Glezer I, Jacobs M, Morgane P (1988) Im- plications for the “initial brain” concept for brain evolution in Cetacea. Behav Brain Sci 11.7-1 16. 19 Morgane PJ, Jacobs MS, McFarland WL (1980) The anatomy of the brain of the bot- tlenose dolphin (Tursiops truncatus). Surface configurations of telencephalon of the bot- tlenose dolphin with comparative anatomical observations in four other cetacean species. Brain Res Bull 5:l-108. 20 Armstrong E, Falk D (1982) Primate Brain Evolution: Methods and Concepts. New York: Plenum Press. 21 Goldman-Rakic PS (1987) Circuitry of pri- mate prefrontal cortex and regulation of bc- havior by representational memory. In Mountcastle VB, Plum F, Geiger SR (eds), Handbook of Physiology. The Nervous System V , pp 373-417. Bethesda: American Physi- ological Society. 22 Byrne R, Whiten A (1988) Machiavellian intelligence: Social Expertise and the Evolution ofIntellect in Monkeys, Apes, and Humans. Ox- ford: Oxford University Press. 23 Dunbar RIM (1993) Coevolution of neocor- tical size, group size and language in humans. Behav Brain Sci 16:681-735. 24 Sawaguchi T, Kudo H (1990) Neecortical development and social structure in primates. Primates 31:283-289. 25 Marino L (1995) The relation between encephalization and diving patterns in Odon- tocetes. Proceedings of the Eleventh Biennial Conference on the Biology of Marine Mam- mals, Orlando, Florida.

0 1997 Wiley Us. Inc.

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Aitchison J (1996) The Seeds of Speech: Language Origin and Evolu- tion. New York Cambridge Univer- sity Press. xi + 281 pp. ISBN

Akazawa T and EJE Szathmary (eds.) (1996) Prehistoric Mongoloid Dispersals. New York: Oxford Uni- versity Press. xi + 389 pp. lSBN 0-

Darnton J (1996) Neanderthal. New York: Random House, Inc. viii + 368 pp. ISBN 0-679-44978-7. $24.00 (cloth). McGrew WC, LF Marchant, and T Nishida (eds.) ( 1 996) Great Ape So- cieties. New York: Cambridge Uni- versity Press. vii + 328 pp. ISBN 0-521-55536. $24.95 (paper). ISBN

Rowe N (1996) The Pictorial Guide to the Living Primates. East Hamp- ton: Pogonias Press. v + 263 pp. ISBN 0-9648825-1-5. $59.95 (pa- per). ISBN 0-9648825-0-7. $79.95 (cloth). To order call 1-800-296- 6310, or write Pogonias Press, 163 Town Lane, East Hampton, NY 11937-5000. Please include $4.95 for postage within the U.S. plus N.Y. sales tax if applicable. On interna- tional orders include $10.00 for pri- ority mail to Canada, Western Europe, and Pacific Rim countries; other countries include $20.00 for air mail. de Waal FBM (1 996) Good Natured: n e Origins of Right and Wrong in Humans and Other Animals. Cam- bridge: Harvard University Press. vii + 296 pp. ISBN 0-674-35660-8. $24.95 (cloth). Lahr MM (1996) The Evolution of Modern Human Diversity: A Study on Cranial Variation. New York: Cambridge University Press. xvi+ 416 pp. ISBN 0-521-47393-4. $74.95 (cloth). Betzig L (ed.) (1996) Human Na- ture: A Critical Reader. New York: Oxford University Press. xvi + 489

0-521 -46246-0. $49.95 (cloth).

19-8523 18-1. $150.00 (cloth).

0-521-55494-2. $64.95 (cloth).

pp. ISBN 0-195-0986-5X. $39.95 (paper). Clark WR (1 996) Sex and the Origins of Death. New York: Oxford Univer- sity Press. xii + 190 pp. ISBN 0-19-

510644-X. $22.00 (cloth). Goldschmidt T (1996) Darwin’s &ampond: Drama in Lake Victoria. Marx-MacDonald S (translator). cambridge: The MlT Press. 274 pp.

Mithen S (1996) The Prehistory of the Mind. New York: Thames and Hudson. 288 pp. ISBN 0-500-05081- 3. $27.50 (cloth). Tainter JA and BB Tainter (eds.) (1 996) Evolving Complexity and En- vironmental Risk in the Prehistoric Southwest. New York: Addison Wesley Longman. xii + 296 pp. ISBN 0-201- 87040-1.34.49 (paper). Gummerman GJ and M Cell-Man (eds) (1996) Understanding Com- plexity i n the Prehistoric Southwest. New York: Addison Wesley Long- man. xii+380 pp. ISBN 0-21-52766- 9. $34.38 (paper). Johanson D and E Blake, principal photography by D Brill (1996) From Lucy to Language: The Record of Human Evolution. New York: Si- mon and Schuster Editions. 272 pp. ISBN 0- 684-81023-9. $50.00 (cloth). Cole M (1996) Cultural Psychology: A Once and Future Discipline. Cam- bridge: Harvard University Press. xvi + 400 pages. ISBN 0-674-17951- X. $29.00 (cloth). Wrangham R and D Peterson (1996) Demonic Males. Boston: Houghton Mifflin Company. 350 pp. ISBN 0- 395-69001-3. $24.95 (cloth). Meikle WE, FC Howell and NG Jablonski (1996) Contemporary Zs- sues in Human Evolution. Wattis Symposium in Anthropology. Memoir 21. San Francisco: Califor- nia Academy of Sciences. vii + 193

(cloth). Tattersall I (1995) The Fossil Trail. New York Oxford University Press. xi + 276 pp. ISBN 0-19-510981-3. $14.95 (paper). Bennet KD (1 997) Evolution and Ecology: The Pace of Life. Cam- bridge: Cambridge University Press. xvii + 241 pp. ISBN 0-521- 39921-1. $24.95 (paper) ISBN 0-

ISBN 0-262-07178-9. $25.00 (cloth).

pp. ISBN 0-940228-45-9. $35.00

521-39028-1 $65.00 (cloth).

Bernore RL, V Fahlbusch, and HW Mittman (eds.) (1996) The Evolu- tion of Western Eurasian Neogene Mammal Faunas. New York: Co- lumbia University Press. ix + 487

(cloth). Greenhalgh S (1997) Situating Fer- tility. Cambridge: Cambridge Uni- versity Press. xv +304 pp. ISBN 0521470447 $64.95 (cloth). Kinzey WG (ed.) (1997) New World Primates: Ecology, Evolution and Behavior. New York: Aldine de Gruyter. xvii + 436 pp. ISBN 0-202- 01186-0. $24.95 (paper). ISBN 0-

Kay RF, RH Madden, F U Cifelli, and JJ Flynn (eds.) (1997) vertebrate Pa- leontology of the Neotropics: The Miocene Fauna o f La Venta, Colom- bia. Washington: Smithsonian In- stitution Press. 496 pp. ISBN

Dunbar R (1 996) Grooming, Gossip, and the Evolution of Language. Cambridge, MA: Harvard Univer- sity Press. 230 pp. ISBN 0-674- 36334-5. $22.95 (cloth). Dunbar R (1995) 7he Trouble with Science. Cambridge, MA: Harvard University Press. vii + 2 13 pp. ISBN 0-674-910 19-2. $14.00 (paper). Richards PW (1996) Tropical Rain Forest. Second Edition. New York: Cambridge University Press. xxiii + 575 pp. ISBN 0521421943. $49.95 (cloth). Foley R (1995) Humans Before Ha- manity. Malden, MA: Blackwell Publisher. vii + 248 pp. ISBN 0-631- 20528-4 $21.95 (paper). Williams GC (1 997) The Pony Fish’s Glow: And Other Clues to Plan and Purpose in Nature. New York: Basic Books, A Division of HarperCollins Publishers. viii + 192 pp. ISBN 0-

Runciman WG, J Maynard Smith and RIM Dunbar (eds.) (1 996) Evo- lution of Social Behaviour Patterns in Primates and Man. Oxford: Ox- ford University Press. vi + 297pp.

(cloth).

pp. ISBN 0-231-08246-0. $92.00

202-01 185-2. $49.95 (cloth).

1-56908-418-X. $80.00 (cloth).

465-07281-X. $20.00 (cloth).

ISBN 0-19-726164-7. $48.00