76 



SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 



[April 24, 



G. The presence of a free centrale in the carpus in all rodents 

 except the Hystricidae and Cadogcnys and the general fusion of the 

 scaphoid and lunar in all except the Bathyergidse, Ctenodoctilidse and 

 LagomorphaJ The centrale is wanting in the carpus of the Typo- 

 theria and the lunar is always free. 



H. The presence of a tibial sesamoid in all the simplicidentate 

 rodents. This is not found in the tarsus of the Typotheria. 



The Typotheria resemble rodents in the elongation of the anterior 

 portion of the skull with the reduction of the incisor-canine-premolar 

 series {cf. Figs. 3 and 4), in the enlargement and often permanent 

 growth of the median incisors (not homologous with the enlarged 

 incisors in rodents, see under A, above), in the development of a 

 mastoid dilation which may be filled with cancellse (Interatheridse) 

 as in many rodents and connected with the 

 auditory bulla, in the shape of the proximal 

 articular surfaces between the radius and ulna, 

 in the broad anteriorly directed transverse 

 processes of the Ivimbar vertebrje and in 

 several other characters of minor importance. 

 In view of the striking differences in struc- 

 ture indicated in the preceding paragraphs, 

 it seems probable that these resemblances are 

 to be explained as instances of convergence. 



4. With the Hyracoidea. — A more or less 

 intimate relationship between the Typotheria 

 and Hyracoidea is commonly assumed but 

 with the complete material now available it is 

 difficult to see on what grounds this hypothesis 

 can be maintained. The hyracoid carpus is 

 arranged on the linear plan with separate 

 centrale while in the tarsus the astralagus is 

 unlike that of any other mammal in possessing a large step-like 

 articulation for the internal tibial malleolus (Fig. 10). 



In striking contrast with hyrax, the carpus in the Typotheria is 

 strongly interlocking without centrale, and the internal tibial mal- 

 leolus is applied to the lateral surface of the astragalus without trace 



' Weber, loc. cit., p. 476. 



Ill 



Fig. 10. Left hind 

 foot of Procavia 

 (Dendrohyrax) arbo- 

 rea, ^ natural size. 

 (No. 365 Princeton 

 University osteologi- 

 cal collection.) 



