270 MAMMALIA. 



of ordinary land mammals ; they also differ in having a more 

 differentiated dentition and a comparatively longer neck ; but, 

 so far as the known parts of the skeleton are concerned, they 

 are as essentially Cetacea as their modern representatives. 

 The results of evolution within the order are observable (i.) in 

 the shortening and deepening of the brain-case, (ii.) in the 

 raising of the external narial opening to the top of the base of 

 the rostrum, (iii.) in the simplification of the teeth, their multi- 

 plication in some groups, their loss in others, and (iv.) eventually 

 in the fusion of the vertebrae of the shortened neck. 



Sub-Order 1. Archceoceti. 



The earliest known Cetacean skulls, occurring more or less 

 fragmentarily in the Eocene Formations of Europe, North 

 Africa, and North America, perhaps also in New Zealand, 

 exhibit less than 44 teeth (the normal number in Eutherian 

 mammals) ; and a few of the hindermost of these teeth are 

 implanted by two distinct roots. The temporal fossae are large, 

 the brain-case being elongated and surmounted by a strong 

 sagittal crest. The nasal bones are well-developed and elon- 

 gated, while the external narial opening is situated on the top 

 of the rostrum at about its middle. Only one generic type has 

 hitherto been recognized with certainty, namely, that commonly 

 known as Zeuglodon. 



FIG. 158. 



Zeuglodon cetoides; upper molar, showing the low serrated crown and the two 

 long roots, about one-fifth nat. size. Eocene; Alabama. 



Zeuglodon (fig. 158). When originally discovered in 1834, fragments 

 of the jaws and teeth of this animal were erroneously ascribed to a reptile 

 under the name of Basilosaurus. The skull, mandible, and dentition are 

 now almost completely known ; vertebrae from most regions of the trunk 

 have also been discovered ; while the scapula, humerus, and dermal plates, 



