36 



THE QUARTERLY REVIEW OF BIOLOGY 



Horn and Cape of Good Hope. The possi- 

 bilities for the dispersal of cetaceans are 

 exceptionally great, in view of the absence 

 of physical barriers, and apparently the 

 chief controlling factor is the food supply. 

 Of the thirty genera of living toothed 

 whales, four are restricted to rivers, occur- 

 ring not only in their estuaries, but 

 occasionally in their tributaries as well. 

 One or two genera, like Sotalia and Steno, 

 are essentially tropical in their distribu- 

 tion and are represented by one or more 

 species in the Atlantic, Pacific, and 

 Indian Oceans. Many odontocete genera 

 are more widely distributed, and some of 

 them, especially the killer (Orcinus), the 

 lesser killer (Psendorca), the common dol- 

 phin (Delphmus), the sperm whale (Phys- 

 eter), and one of the beaked whales 

 (Ziphius) are practically cosmopolitan. 

 The bowhead (Balaena mysticetus), which 

 frequents the Arctic seas, and the pigmy 

 whale (Neobalaena marginata) of New 

 Zealand waters appear to be restricted in 

 their dispersal by climatic conditions. 

 Some of the whalebone whales are without 

 doubt the largest mammals that have 

 ever lived. No mammal, not even the 

 zeuglodonts, has attained the proportions 

 of the blue or sulphur bottom whale 

 (Sibbaldus), individuals of which have 

 measured 98 feet in length. Not all of the 

 living whalebone whales are so large, and 

 one of the smallest is the pigmy whale 

 (Neobalaena), which rarely reaches a 

 length of more than zo feet. 



The geological and geographical distri- 

 bution of whales assembled in table 1 

 shows many gaps, and the discontinuous 

 distribution in many instances is due either 

 to the inadequacy of the material col- 

 lected or to lack of effort in areas where 

 material could be secured. 



ARCHAEOCETI 



The oldest known relatives of the 

 typical cetaceans are found in some of the 



oldest Tertiary rocks; that is in those that 

 correspond in age to the early part of the 

 Age of Mammals. Even these archaic 

 mammals were well specialized in many 

 respects for a pelagic life. It so happened 

 that the first notice of these mammals was 

 based on a few fragments found in the 

 Eocene of southeastern Caldwell Parish, 

 Louisiana, near the Ouachita River. Dr. 

 Harlan (1834) believed that they belonged 

 to a giant reptile, to which he gave the 

 name Basilosanrus, but Owen (1839) was 

 able to demonstrate that they were a part 

 of some colossal mammal, for which he 

 proposed to substitute the name Zeuglodon, 

 in allusion to the yoke-like appearance of 

 the cheek teeth. So far as known the 

 oldest zeuglodonts had already acquired a 

 complicated organ for hearing, they 

 retained a well developed olfactory ap- 

 paratus, but their brain structure as shown 

 by casts indicates that their sight was 

 defective. Although they tried out 

 braincases of somewhat different pro- 

 portions, the main path of their evolu- 

 tionary advance seems to have been limited 

 to a remodeling of the cheek teeth. 

 Complete skeletons of the early Eocene 

 zeuglodonts have never been found, but 

 those of the Upper Eocene had lost the 

 functional use of their hind limbs, for the 

 femur was vestigial and the pelvic girdle 

 atrophied. 



It is neither desirable nor possible to 

 recount in detail here the various views 

 that have been advanced regarding the 

 affinities of the zeuglodonts. In recent 

 years the concept that they are related to 

 if not descended from the primitive 

 insectivore-carnivore stock has had wide 

 acceptance. Morphologically they seem 

 relatively near to the typical whales and 

 porpoises, although it is not necessary to 

 assume that any known zeuglodont is 

 ancestral to some particular kind of whale, 

 for the zeuglodont skull in its general 

 structure appears to be divergent from 



