igoS.J 



SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 77 



of the supporting shelf (Figs. 5 and 7, A). The flat astragalar 

 head in the Hyracoidea and the articulation of the fibula with the 

 astragalus instead of with the calcaneum are additional points of 

 difference, all of which are more than sufficient to offset similarities 

 in skull structure which are confined to a few points, such as the 

 cancellous dilation of the mastoid, the shape of the posterior border 

 of the palate, and the increase in depth posteriorly of the mandible. 

 In the Hyracoidea the molar takes part in forming the outer por- 

 tion of the glenoid cavity, the parietal enters into the postorbital 

 process and the base of the coronoid just back of the last lower 

 molar is perforated by a large foramen, a superior branch of the 

 alveolar canal. None of these characters are exhibited by the Typo- 

 theria. In the hyracoid dentition, the first upper incisor is a per- 

 sistently growing downwardly curved tusk of triangular cross- 

 section. In some of the Typotheria this tooth may grow persis- 

 tently but it is always antero-posteriorly compressed, transversely 

 expanded and modified for cropping, never appearing as a tusk. 

 The molars of the Hyracoidea are lophoselenodont and either 

 brachyodont or short hypsodont while in the Typotheria they are 

 extremely hypsodont, developing roots only in the deciduous series. 

 The crown pattern of the hyracoidean molar bears more resemblance 

 to that of some of the early horses and rhinoceroses than to the 

 molar pattern in the least specialized of the typotheres (Figs. 8 

 and 9). 



The so-called hyracoids from the Fayum Province of Egypt 

 {SagJiatheriunt, Megalohyrax) are as yet known only from frag- 

 ments of the skull and dentition but, so far as the available material 

 permits comparison, resemble the modern Hyracoidea and not the 

 Typotheria which would probably not be the case if the two orders 

 were related as it would naturally be expected that a closer simi- 

 larity should exist between the Eocene and Aliocene representatives 

 of an order than between the latter and the recent forms. All the 

 Eg}'ptian hyracoids have the base of the coronoid perforated by a 

 branch of the dental canal as in the recent forms* and unlike the 

 Typotheria. 



* Communicated by Mr. Walter Granger, of the American Museum of 

 Natural History. 



