NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 1 5 



in common with the spinous processes, widely separated from each 

 other. Thus in Zeuglodon the body has acquired an altogether 

 peculiar length, putting one in mind of the snakes. The posterior 

 thoracic vertebrae seem to have developed considerable transverse 

 processes which bore the ribs on their extremities. In size also 

 Zeuglodon went further than its relatives. Of the hind limb there 

 is known a small, quite atrophied pelvic bone with articular surface 

 for the femur, and an even more degenerated little rod-shaped femur." 



The genera of Zeuglodonts together form the section Arcliseoceti, 

 the source from which all the higher cetaceans have originated. Pro- 

 tocetus has scarcely a single peculiarity, apart from its large size, 

 that one would not expect to find in an ancestral stock for the higher 

 whales. The same is true of Proseuglodon. On the other hand 

 Zeuglodon, a descendant of Proseuglodon, has followed its own line 

 away from the starting point of the other whales, deviating particu- 

 larly in its remarkable vertebrae. 



The peculiarities which especially place the Zeuglodonts lower than 

 all other cetaceans are that the teeth are still present in the typical 

 number, and that the braincase is not telescoped and shortened. Of 

 all other whales it holds good that they, so far as they are known. 

 have the number of teeth raised above the typical (or that they are 

 descended from cetacea in which it had been raised), and that they 

 have the braincase more or less compressed antero-posteriorly. As 

 regards the form of the teeth Protocetus no doubt stands lower than 

 all other cetaceans ; but Proseuglodon and Zeuglodon are in this 

 respect scarcely more primitive than the lowest members of the higher 

 families. Of all the many other primitive characters that are found 

 in the Zeuglodonts some are, it is true, no longer to be found in the 

 higher families, not even among the extinct lowest forms ; but for 

 most of them this does not hold good. 



Zeuglodontidae " [Basilosauridse] . 



I. Crowns of cheekteeth with smooth, not serrate, margins. 

 Protocetus. 

 IL Most of the cheekteeth have serrate anterior and posterior 

 margins to the crowns. 



A. Centra of thoracic, lumbar, and caudal vertebrae not 



elongated. 

 Proseuglodon. 



B. Centra of posterior thoracic, of lumbar and caudal 



vertebrae elongated. 

 Zeuglodon [Basilosaurus'j. 



