272 MAMMALIA. 



Prosqualodon. Known only by an imperfect skull and mandible, 

 which is identical in most respects with that of a modern Odoutocete but 

 shows the small triangular nasal bones carried on a projecting ridge of the 

 frontals in such a manner that they slightly roof the nasal chamber. The 

 mandibular symphysis is comparatively short. So far as preserved, the 

 teeth resemble those of Squalodon, but only five or six of the hinder ones 

 are implanted by a double root. The species is named P. australis. 



Squalodon. The skull and dentition of this genus are completely 

 known by fine specimens discovered in the Miocene of France and Bavaria. 

 The skull precisely resembles that of the existing Odontoceti, with the 

 external narial opening at the base of the rostrum, and the nasal bones 

 quite rudimentary, not roofing the nasal chamber. The teeth are more or 

 less enamelled, very similar in form to those of Zeuglodon but more 

 numerous and in closer series. In the best-known species, S. bariensis, 

 there are three simple conical incisors in the premaxilla, while a similar 

 tooth at the anterior end of the maxilla may be regarded as a canine. 

 Then follow four more compressed, but also single-rooted teeth, which 

 may be termed premolars. Finally, there are seven two-rooted molars. 

 The teeth of the mandible are similar, and alternate with those of the 

 upper jaw ; the total number of teeth being thus no less than 60. The 

 symphysis of the mandible is very long, extending backwards beyond the 

 "premolars." Teeth and other fragments are known from the marine 

 Miocene and Pliocene strata throughout Europe, and have been identified 

 also in North America and Australia. 



Most of the remains of the typical Odontoceti found in the 

 Miocene and Pliocene formations of Europe and North America 

 are too fragmentary for satisfactory discussion ; but some fine 

 skulls from the Marine Patagonian Formation (supposed Mio- 

 cene) of the Argentine Republic, South America, exhibit all 

 the principal cranial and dental characters of these early Ceta- 

 cean types. One of these (Argyrocetus} is interesting as being 

 bilaterally symmetrical and having functional nasal bones; 

 hence such a form as might have easily evolved into the skull 

 of a whalebone whale. Another (referred to Physodon] is the 

 skull of a small sperm whale, differing from the modern forms 

 of this type in still retaining teeth in the upper jaw, and having 

 all the teeth capped with enamel. 



Argyrocetus. This genus is founded on a unique bilaterally-sym- 

 metrical skull and mandible about a metre in length, from the Patagonian 

 Formation of Chubut, and specifically named A. patagonicus. The occi- 

 pital condyles, instead of exhibiting the adpressed form of those in the 

 existing Platanistidse and Delphinidse, are as prominent as in any ordinary 



