OBSEKYATIONS OX THE DENTITION OF CHRYSOCHLORIS. 183 



cusps. Phascolestes has also neai-ly the same type of 

 lower molar. The molai's of the Oligocene Insectivores 

 Apter nodus and Micropternodus are also essentially 

 similar, and even those of PaUeosinopa and Pala^onictis 

 differ but slightly. When we compare the lower molars with 

 those of such recent animals as Erin ace us, Dasyurus, or 

 Perameles, we find that the only difference is that in these 

 the talonid is well developed and has a pair of large cusps. 

 Whether the Chrysochloris tooth is degenerate or primitive 

 the parts are pretty certainly homologous with those of 

 Amphitherium, and the trigonid is certainly homologous 

 with the anterior part of the molars of Dasyurus. 



In the upper teeth we meet with a number of difficulties. 

 As in the lower jaw we seem to have no difficulty in tracing 

 the main cusp from the incisors along to the molars. Nay, 

 we even see how a simple conical canine, such as is present in 

 C. hottentota, may become triangular by the development 

 of a postero-external cusp, and nearly molariform in C. 

 obtusirostris. The hrst premolar in most species is like 

 the canine in C. obtusirostris, and the second differs by 

 the addition of an antero-external cusp and a small interual 

 ledge. There is thus formed a trigon of three cusps with an 

 internal basal one, and the question arises, what ai*e they ? 

 It seeilis almost certain that the main cusp is homologous in 

 each of the teeth, for the main cusp in the triangular canine 

 of C. obtusirostris is manifestly the homologue of the 

 conical canine in C. hottentota, and we may therefore 

 safely conclude that the main cusp in the second and third 

 premolars is the fundamental cusp of the tooth, and one 

 would like to call it the " protocone." Unfortunately, when 

 we come to the molars we find that the term " protocone " has 

 been applied to the small basal cusp, and it is difficult to see 

 how the confusion is to be avoided. 



According to the older Cope-Osborn theory the superior 

 ]n-emolars followed a different order of cusp addition from 

 the molars, and quite different names Avere given to tlie cusps. 

 Thus, the antero-internal cusp of the first molar was called 



