2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J2 



6th cusp [protocone] was reduced or absent. All of the upper molari- 

 form teeth including the third premolar still had the inner root, 

 though this was in process of reduction. All the teeth of the typical 

 [eutherian] dentition were present, ii in each jaw [44 in all]. The 

 jaws were elongated in harmony with the long, well-developed tooth- 

 rows. The temporal fossa was very large, widened out by a powerful 

 temporal muscle. It was bounded by a high sagittal crest, by a 

 strong, backward-projecting occipital crest, and by an abruptly out- 

 standing, posteriorly heavy processus zygomaticus squamce. 



In addition to these peculiarities the most primitive whales had two 

 high characters which were perhaps inherited from the Hysenodonts ; 

 at any rate they are to be found in the latter group, though less pro- 

 nounced : a rather large supraorbital process, and a bony palate 

 lengthened backward far under the posterior nares. 



Radical alterations have taken place during the change from 

 Hysenodont-like carnivores to true whales. In many of the mam- 

 malian groups there have arisen forms modified for life in the water ; 

 but no other aquatic mammals are modified to the same degree as the 

 cetaceans, nor has any other become so exclusively aquatic ; only to 

 breathe do they raise the nose above the water in which they other- 

 wise are hidden. 



The cetacea have used the tail as the chief implement of locomo- 

 tion ; the hind limbs are put wholly out of service ; the fore limbs are 

 scarcely used for much else than steering and balancing. 



The tail becomes enormous, long and thick, powerfully muscled. 

 It is formed in agreement with the manner in which it is wielded: 

 with strokes from side to side, or up and down, or with a sculling 

 motion. Throughout most of its extent it becomes compressed, but 

 at the tip it acquires a powerful, horizontal, caudal fin constructed of 

 skin folds (not present in quite young embryos of recent cetaceans). 

 At the front of its upper margin, in the region where the tail joins 

 the back, there may occur a special erect skin fold in the form of a 

 longitudinal crest, a dorsal fin. Most of the caudal vertebrse lose the 

 atrophied appearance which they have in primitive mammals ; they 

 acquire powerful centra, heavy, flat-outspread transverse processes, 

 high dorsal arches with large, compressed spinous process, and 

 articular processes which are distinct though not mutually fitting 

 together. The ventral arches with the inferior spinous processes 

 become so large that they approach the upper arches in size. Only 

 the outermost caudal vertebrae, which lie almost inclosed in the caudal 

 fin, retain the degenerate character. The tail has an influence on the 



