NOTOUNGULATA 



227 



becomes a long, oblique protoloph, and the hypocone a short, straight meta- 

 loph. Frequently also crests or crochet-like spurs are formed on these molars, 

 and a parastyle appears on the front surface, so that these teeth display a 

 certain similarity to the upper molars of the rhinoceros. On the posterior 

 surface still another ridge may develop from an extra cusp. The molars of 

 primitive forms are much broader than long. The lower molars consist of 

 two outer semilunar ridges, of which the anterior one is much shorter than 

 the posterior, and is formed from two more or less compressed inner cusps. 

 The foremost of these is joined inwardly with the posterior portion of the 

 first semilunar ridge, whereas the hinder one lies opposite the second semi- 

 lunar ridge, frequently remaining entirely isolated. These molars have a 

 general resemblance to those of the Litopterna, but they pass through no 

 well-defined bunodont stage in their development. The conformation of the 



Fig. 309. 



Upper and lower molars of Notoungulata, showing grinding surface. Upper molars : A, Pleurostylodon. 

 B, Protypotherium, '■i/i. C, Rhynchippus. D, Rhynchippus,. M3 from behind. Lower molars : A, Pleurostylodon. 

 B, Proty pother iura, ~/i. C, ilorphippus. C, crochet ; Cr, crest ; E, ectoloph ; M, metaloph ; P, protoloph ; 

 Ps, parastyle ; a-, anterior crescent ; md, metaconid ; pc, posterior crescent ; pp, posterior pillar. 



incisors, canines and premolars gradually leads up to the molariform tooth. 

 Often one or two pairs of incisors are greatly enlarged, or, together with the 

 canines, disappear entirely, or again these latter may develop into tusk-like 

 eye-teeth, in which case one to three premolars are lost. The teeth are 

 originally brachyodont, later becoming hypsodont, and in the final stage 

 they become prismatic and lose their roots entirely (Fig. 310), although even 

 in this case the premolars are considerably lower than the molars. 



As indicated by Santiago Roth, the chief distinguishing characteristic of 

 the Notoungulata consists in the broadening and swelling up of the mastoid 

 region of the skull, and the joining of this cavity, frequently filled out with 

 spongy, bony tissue, with the tympanic bulla. Even though this distinction 

 is always found among the older forms, yet there are many marked stages in 

 the development of the swelling of the mastoid region, so that there can 

 be no good reason for excluding the Astrapotherioidea from the Notoungulata 

 because they lack this characteristic. 



The bones of the extremities show considerable variation and modification 



