68 SINCLAIR— THE SANTA CRUZ TYPOTHERIA. [April 24, 



nent feature of the back of the skull is the greatly distended mastoid 

 tract which may either be filled with cancellse or lodge a large cavity. 

 In either case there is direct communication with the tympanic bulla 

 and the dilation appears to have functioned as a secondary resonator, 

 perhaps associated with nocturnal habits. The palate is concave 

 throughout, terminating posteriorly in a pair of stout processes. 

 The mandible is heavy and deep, without trace of suture in the 

 firmly fused symphysis. 



2. Dentition. — Beginning with the normal incisor formula in 

 Protypothcriiim (Fig. i) the Santa Cruz typotheres show a well- 

 marked tendency toward an increase in size of the median incisors 

 at the expense of the lateral incisors, canine and anterior premolar 

 until the extreme stage of reduction in Pachyrukhos (Fig. 4) is 

 attained. The teeth undergoing elimination are reduced to simple 

 cylinders. It is not to be understood that Protypotherium, Intera- 

 therium, Hegetotheriiim and Pachyrukhos constitute a phyletic 

 series because they represent successive stages in the process of 

 dental reduction associated with the hypertrophy of the median in- 

 cisors. As already indicated in the key to the genera, two divergent 

 lines are represented and not a single progressive series. A rather 

 curious feature of the lower incisors in Protypotherium is the pres- 

 ence in the first and second of a deep median cleft producing a 

 fork-like structure recalling a somewhat similar division of the lower 

 incisor crowns in the Hyracoidea. In all the Santa Cruz typotheres 

 the enamel layer on the enlarged incisors tends to be confined to the 

 anterior surface of the crown. The molars in all the genera are 

 constructed on much the same plan but only in Protypotherium are 

 absolutely unworn teeth known, consisting essentially of a broadly 

 concave ectoloph {e, Fig. 8, ^) and a pair of crescents with the con- 

 vexity directed inward (oc, pc, Fig. 8, A), of which the anterior 

 horns are fused with the ectoloph inclosing a reentrant. A crista- 

 like ridge from the ectoloph {c, Fig. 8, ^) is separated from the 

 anterior crescent by a deep notch. A slight ridge {pp, Fig. %, A) 

 blocks the shallow valley inclosed by the posterior crescent. As the 

 tooth wears the antero-external angle of the crown elongates and is 

 channeled by a shallow groove producing the ridges noted in the 

 kev to the genera. 



