26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



jaw the symphysis menti becomes very long, and the rami of the 

 mandible are inclined to grow together. The teeth that lie at the 

 front of the jaws are inclined to increase in size, probably because the 

 tips of the jaws come to be the most used part of the forceps. In 

 another respect also the known Platanistids stand high: the sides of 

 the facial depression are inclined to grow upward. Perhaps they also 

 stand high in the tendency of the pterygoid to widen itself out 

 unusually far backward in the outer wall of the air-sac behind the 

 palate, reaching back to the squamosal ; a circumstance that may call 

 to mind both lower and higher whales, Balsenids and Physeterids.^* 



The genus of Platanistids which has removed itself least from 

 Squalodon appears to be the Tertiary South American Pontistes, 

 which is known from most of the skull. In relation to one or another 

 of its nearest allies it has the following primitive characters : the teeth, 

 judging from the alveoli, were small, simply formed, the anterior not 

 enlarged ; the toothrows stand rather distant from each other, as the 

 palate is relatively broad ; the outer margin of the facial depression, 

 especially the longitudinal crest on the maxillary above the orbit, is 

 relatively low. A character which must be considered advanced in 

 comparison with the nearest relatives is the specially large number of 

 . teeth, about 40 in each jaw if one judges rightly from the fragments 

 of toothrows that have been found. 



Near to Pontistes but on a slightly higher level is Pontoporia 

 {Stenodelphis). The teeth have become smaller but more numerous, 

 about 55 in each jaw. Those in front have a slight tendency to be 

 enlarged. The toothrows are placed nearer together and the palate 

 is narrower. A high specialization, which also holds good for the 

 other recent members of the family, is the complete absence of the 

 olecranon. 



Lipotcs (known from external characters, skull, and cervical verte- 

 brae) and Inia are near relatives of Pontoporia. Their face is shorter, 

 the number of teeth is less (about 30 in each jaw in the former, about 

 26 in the latter), the anterior teeth show scarcely any tendency to be 

 enlarged. It might appear as if the two genera were, in these char- 

 acters, less specialized ; but the explanation is presumably another. 

 The two genera most likely originated from forms that more nearly 

 resembled Pontoporia, and that had the strongly narrowed palate and 

 numerous small simply conical teeth, although not so many as in 

 Pontoporia. Lipotcs and Inia appear to have used the teeth in a 

 special manner, most probably for the crushing of food, and the teeth 

 have therefore regained some of their earlier strength, have grown 



