AQUATIC MAMMALS 



and very little is known about them. The tail took the form of two lat- 

 eral pointed lobes and the flippers were relatively small and said to have 

 been covered with short, coarse hairs, although the body was naked. The 

 cervical vertebrae numbered seven, and they probably lacked nasal bones. 

 There were no teeth in the adult, their places being taken by horny oral 

 plates. 



The sirenians are commonly believed to have been derived from the 

 progenitors of the proboscidian stock. Known remains date back to 

 the Eocene, and even then they were highly modified for an aquatic 

 life. They are not as progressively specialized as the whales but their 

 sedentary existence with no need for seeking active food is not as stimu- 

 lating for rapid evolutionary change and their aquatic habits are un- 

 doubtedly of great antiquity. 



CETACEA 



Archaecoceti or zeuglodonts were essentially whale-like in body form 

 and general details, but were less specialized in some respects. Thus 

 the anterior limb was less modified and the skull had few of the char- 

 acteristics which we associate with modern whales. Externally the hind 

 limbs had disappeared, however. According to Kellogg (1928) zeuglo- 

 donts are first known from the Eocene, but were evidently a rather un- 

 successful experiment for they suddenly disappeared in the Upper Oli- 

 gocene. Although the narial aperture of the skull shows considerable 

 retrogression, the bones exhibit no indication of the telescoping that is 

 so characteristic of the modern whales. The dentition of the more 

 primitive sorts resembles to a considerable extent that of certain creo- 

 donts, but this is believed to be only superficial, and the skull is re- 

 markably suggestive in general conformation of some Cretaceous insec- 

 tivores, and indeed modern ones as well. The majority of paleontolo- 

 gists, I believe, consider the zeuglodonts and modern whales to have had 

 a common ancestry, but that it is impossible that the latter could have 

 descended from any sort of zeuglodont now known. Most likely this an- 

 cestor was very different from either, the modern whales diverging in one 

 direction and the zeuglodonts branching at another tangent in a direc- 

 tion that proved unsuccessful. It is interesting to note, as Kellogg says, 

 that "periotic bones of both the whalebone and toothed whale type have 

 been found attached to skulls of zeuglodonts," and, I may add, no other 

 mammal known, either living or extinct. 



Of those zeuglodonts so far known there are two extremes of body 

 type. One of these is represented by Basilosaurus, with very long tail 



[41] 



