SUBORDER A 



TYPOTHERIA 



229 



inflation of the squamoso- mastoid region in this group attains its highest 

 stage among the Notoungulata in Pachyrucos, but the character is by no 

 means developed to an equal degree among diff'erent genera. This capsule, 

 at times hollow and at times filled with a cellular osseous tissue, is connected 

 with the tympanic bulla, and encloses the broad, upwardly directed auditory 

 meatus. The palate often extends far behind the last molars. The high 

 mandible forms a strong symphysis, and is very like that of Hyrax in that it 

 has a broad, ascending ramus and a short coronoid process. 



The dentition is ordinarily complete with „ , , ^ teeth in compact rows ; 



a wide diastema is sometimes formed by the disappearance of the outer 

 incisors, the canines, and of one or two premolars. In the older forms all the 

 teeth are brachyodont ; in later forms this applies only to the milk-teeth. 

 The lower incisors often have a deep indent in the middle. The upper 

 I^ and the lower 1-^ and 1^ become larger and longer and more rodent-like, 

 although they are antero-posteriorly compressed rather than transversely. 

 The other incisors, the canines and premolars become very gradually molari- 

 form, or else the incisors and canines either degenerate into mere pegs or are 

 lacking entirely. The premolars are usually simpler than the molars, since 

 their posterior moiety usually remains smaller than the anterior one. The 

 upper premolars and molars become bent and inwardly directed prismatic 

 teeth, while those of the lower jaw are outwardly directed. The upper molars 

 consist of an ectoloph, which has a projecting ridge, an oblique protoloph, a 

 somewhat shorter metaloph, a crest and a posterior pillar. The lower molars 

 have two outer crescents and two strongly compressed inner pillars, which at 

 times fuse to form a well-defined ridge. The lower third molar is always 

 much larger than the second. The molars of the Typotheria are distinguished 

 from those of the Toxodontia by the posterior and anterior moieties being 

 of nearly equal size, and by the rapid disappearance of the valleys in the 

 wearing down of the teeth. The upper molars are further distinguished by 

 the noticeable elongation of the grinding surface and by the secondary spurs. 

 The teeth are often provided with a thin layer of cement. 



A 



Fig. 311. 



Typotherium cristatum Serres. 

 a, Acromial process ; c, cora- 

 coid process. (After Gervais.) 



Frotypotherium. A, Calcaneum, anterior aspect, as, Facet 

 for astragalus ; ch, for cuboid ; p, for fibula. B, C, Astragalus, 

 anterior and posterior aspects. n, Facet for navicular ; 

 tr, trochlea for tibia. 



The dorsal vertebrae are fifteen in number, the lumbars seven, and the 

 sacrals five. This is true at least for the Typotheria of the Miocene of Santa 

 Cruz. The tail seems to have been long in most cases. The scapula (Fig. 311) 

 has an acromial and coracoid process, and articulates, no doubt, with a well- 



