1908. J 



SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 69 



In the lower molars the convexity of the crescents is reversed so 

 that the reentrant fold is external (Figs, i, 3, 9, /i). A prominent 

 lobe spanning the arc of the posterior crescent {pp, Fig. <^, A) is not 

 peculiar to the teeth of the Typotheria alone, but is present also in 

 Nesodon (Fig. g, B), Astrapotherium, Theosodon and other extinct 

 ungulates from South America. In the last lower molar the devel- 

 opment of the third lobe present in Inter athermm is accomplished 

 by the deepening of the shallow groove indicated in Protypotheriiini 

 at the point marked pc in Fig. 9, A. 



As mentioned in the generic key the premolars are sometimes 

 molariform and sometimes not, diitering from the molars in the 

 latter case in having the anterior crescentic lobe smaller than the 

 posterior. 



Roots are developed only in the deciduous molars but as these 

 have been observed only in Profypotlieriitjii and Interatherium it is 



Fig. 4. Skull of Pachyrukhos inoyani Ameghino,. side view, three fourths the 

 natural size. (Reconstructed from several specimens.) 



not altogether certain whether this character is of family or sub- 

 ordinal value. So far as can be ascertained the crown pattern seems 

 to have been the same in the deciduous and permanent series, the 

 milk teeth resembling their successors. The order of replacement 

 seems to have been the normal one. 



A thin layer of cement is usually observable on the molars and 

 premolars of all the genera. 



3. Axial Skeleton. — The dorso-lumbar vertebral formula in 

 Interatherium is twenty-two, of which fifteen are dorsals. It was 

 probably the same in Protypotherium but in Pachyrukhos eight lum- 

 bars are present. Five vertebras are coossified, in the sacral com- 

 plex of which three are true sacrals in contact with the ilium and 



