UPPER CRETACEOUS MARSUPIALIA 



119 



b b,c,c 



£P 



do not meet each other, but run to the bases of their respective primary cusps, the para- 

 cone and metacone. The latter are higher than the protocone and more slender. They 

 have sharp anterior and posterior ridges, but are only slightly compressed transversely. 

 The external cingulum is very well developed. From a study of very numerous Cre- 

 taceous didelphid upper molars as well as of a large series of later didelphids from all 

 known horizons, it is apparent that the ancestral 

 didelphid had typically five stylar cusps, the 

 history of each of which may usually be fol- 

 lowed with some clarity. Bensley's notation for 

 the didelphid styles, being based on much later 

 forms only, is not readily applicable and the 

 writer proposes a modification of it, designating 

 the five primary styles A, B, C, D, and E respec- 

 tively, from anterior to posterior. (See Fig. 43.) 

 In the present tooth A is anterior and very 

 slightly external to the paracone, while C and 

 D are small and ridge-like. 



The following are the most striking varia- 

 tions seen in teeth of this general tj^e : 



1. The width varies from about 2.5 mm. to 

 about 6.0 mm. 



2. The proportions vary. Some are about as 

 broad as long, others much broader than long. 

 This is largely due to position in the dental 

 series, M^ being much more transverse than M^ 



3. Some molars are much more oblique 

 than others — also due largely to position. 



4. The external border may be a straight 

 oblique line or it may run almost anteroposterior 

 opposite the pa and then turn rather abruptly 

 outward. The cingulum may be obsolete oppo- 

 site the pa. The latter is due, at least in part, to position in the series also, as the cin- 

 gulum opposite the pa is less developed on M*"^ than on M*"*. 



5. The stylar cusps vary considerably. A is always present and is only slightly 

 external to the pa save on M\ B is never well developed, while C and D are about 

 equally developed and usually form ridge-like crests parallel to the outer border. In 

 some cases, however, they are cone-like and this marked difference is probably of sys- 

 tematic value. E is always present save on M*. 



On teeth supposed to be first molars, the pa and me are approximated to the pr, 

 crowding the conules, B is practically absent, C is represented, if at all, only by small 

 median tubercles, and E is slightly coarser than usual. 



Last molars, such as Y.P.M. No. 13650, differ radically from the above, but in 

 just the same way as M* of Didelfkis differs from the preceding teeth. W is strongly 



Fig. 43. Didelphid upper molars. A, 

 Bensley's stylar designations. B,Pedio- 

 mys, showing five primitive stylar cusps 

 and their suggested designations. C, 

 last upper molar of Pediomys. D, Eclo- 

 conodon. E, Alphadon. F, Didelphis. 



