1892.] NATURAL SCIENCES OF PHILADELPHIA, 429 



in the different sjDecies, but no deuteroconid ; in H. xiphodon and 

 H. Uneaius the talon (metaconid) of P'^ and *^ is separated by a 

 median depression into long external and internal ridges, a ten- 

 dency which also appears in many primitive artiodactyls. 



In all the members of the Periptychidae there is a more or less 

 decided tendency for the premolars to become very massive and in 

 many species they are distinctly larger than the molars. 



VII. HYRACOIDEA. 



The discovery, recently announced by Professor Marsh (No. 6), 

 of the foot-structure of Menlxcotherium justifies, in my opinion, 

 the removal of that genus and family from the Condylarthra to the 

 Hyracoidea. Meniscotherium, low as is its place in the geological 

 scale, already exhibits a considerable degree of premolar complica- 

 tion and although we cannot assert that it is the ancestor of Procavia 

 {Hyrax, see Oldfield Thomas, No. 13), yet it will suffice to show 

 the stages of dental evolution. P^ is small and simple, implanted 

 by a single fang; P^ has a similar but somewhat larger crown, and 

 is inserted by two roots. ^ has a deuterocone of more or less crescen- 

 tic pattern, while P* is almost molariform. The proto- and trito- 

 cones are of nearly equal size, are compressed and so joined together 

 as to make a wall. The deuterocone is a large conical tubercle, and 

 the anterior and posterior conules are present, thus forming a tooth 

 which has almost exactly the same construction as "^ which lacks 

 the hypocone, as P^ does the tetartocone. There is some variation in 

 the construction of this tooth, in the presence or absence of a ridge 

 upon the external wall of the crown, marking the separation between 

 the proto- and tritocones, and also in the condition of the cingulum. 



In the lower jaw the premolars are very small and simple. F 

 has a minute heel and the fangs are fused together. F is similar^ 

 but slightly larger. P^ is molariform and consists of two crescents, 

 the composition of which is as follows : the protoconid sends a 

 curved crest inward and forward to what may or may not be the 

 paraconid, and a straight crest inwardly to the deuteroconid ; the 

 posterior crescent is likewise formed by two ridges which pass from 

 the metaconid to the tetarto- and deuteroconids respectively. 



In Procavia {Hyrax) the premolars have all assumed a molar 

 pattern and the lophodont condition is more completely attained, 

 though in the unworn tooth the separate elements may still be made 

 out, which show that the premolars are constructed essentially like 

 the hindermost of the series in Meniscotherium. 



