438 



PROCEEDINGS OF THE ACADEMY OF 



[1892. 



tlie paraconid is enlarged nearly to the size of the protoconid and 

 on the inner side a conical cusp is added, the tooth thus consisting 

 of three pairs of cusps. A similar p* occurs in Dichodon. 



Fig. 8. 



Fourth upper and lower premolars of leftside. 1, Dicotyhs torquaius ; 2, 

 Thinohy us lentils ; Z, Perchcerus probiis ; ^, Fantolestes brachystomus. Letters 

 as in the preceding figures. 



Among the Suina the peccaries offer the most interesting series. 

 The White River and John Day forerunners of that group, Perchoerus 

 and Thinohyus, have extremely simple premolars ; P4 consists of proto- 

 and deuterocones only, and in p the only addition is the low and 

 broad metaconid. In Dlcotyles the last premolars are nearly or 

 quite like the molars. In P4 the tritocone is first added and then 

 the tetartocone, the latter not always occurring, even in the recent spe- 

 cies. In p^ the order of events is similar, first the deuteroconid is 

 added and then the tetartoconid, the latter element and the meta- 

 conid being at the same time more or less elevated. The curious 

 Listriodon, a true pig, exhibits a similar condition ; P4 has devel- 

 oped a tritocone, equal in size to the protocone, and £3 has a large 

 deuterocone. On the other hand, p* has added only the deutero- 

 conid, the talon consisting only of the metaconid. 



It might seem at first sight that many artiodactyls presented an 

 important divergence from the normal scheme of premolar evolu- 

 tion in the way in which the inner crescent of the anterior 

 upper teeth is developed, viz., by the coalescence of anterior and 

 posterior ridges. This is seen in Procamelus, several of the Oreo- 

 donttdce, in some specimens of Palceomenjx, and other forms. But, 

 on the other hand, a careful comparison of the various series will 

 show that the departure from the ordinary course of development 

 is not so radical as it seems. The posterior crest undoubtedly rep- 



