1887.] -'-"*^ [Osborn. 



The Triassie Mammals, Dromatlieriam and Microconodon. 



By Henry F. Osborn, Sc. D. 



{Read before the American Philosophical Society, April 15, 18S7.) 



The mammaliaa jaws discovered by Professor Emmons in the Upper 

 Triassie beds of North Carolina, and ascribed to a single genus, Droma- 

 therium, were recently examined by the writer and found to belong to 

 separate genera. The type mandible of Dromatherium is preserved in the 

 Williams College Museum, and differ widely from the mandible preserved 

 in the Museum of the Philadelphia Academy. These differences have 

 already been pointed out,* but require to be more fully stated, as both 

 Professors Marsh and Cope have expressed doubts as to the distinct sepa- 

 ration of these genera. The accompanying lithographic figures also bring 

 out the characteristic features of these mandibles much more fully than in 

 the pen drawings which accompanied the earlier description. 



In many respects these genera agree with each other, and stand sepa- 

 rate from the Jurassic mammals of both England and America. There is, 

 first, a considerable diastema behind the canine, a very rare feature in the 

 division of Mesozoic mammals to which these genera belong, although 

 always present in the division to which Plagianlax and its allies belong, 

 viz., the sub-order Multituberculata Cope. 



Dromatherimn has three premolars and seven molars, but the number in 

 Microconodon is quite uncertain, as only four of the series are preserved. 

 The molars agree in one particular, which separates them widely from 

 other Mesozoic genera, viz., in the imperfect division of the fangs. This 

 division is indicated merely by a depression at the base of the crown, as 

 in the genus Dimetrodon, among the Theromorph reptiles. 



In all other respects these mandibular rami differ widely. The Micro- 

 conodon ramus is two-thirds the length of that of Dromatherium ; it is flat- 

 tened and slender, with a nearly straight lower border beneath the molar 

 alveoli, and a characteristic depression of the border which possibly rep- 

 resents the angle of the jaw as in Prof. Owen's genus Peraimis. The 

 coronoid process is low and the vertical diameter of the jaw at this point 

 is very narrow. This ramus offers a great contrast to that of Dromatheri- 

 um, which is very stout and convex with a thick lower border, pro- 

 jecting widely from the matrix, an elevated coronoid process and has the 

 curvature of the lower border unbroken by any downward projection. If 

 these differences may be given merely a specific value, and attributed in 

 part to the fact that the Microconodon jaw is seen upon the outer surface, 

 and that of Dromatherium upon the inner surface, let us compare closely 

 the teeth in the two genera., Unfortunately the canine and incisors of the 



* Proceedings of the Academy of Natural Sciences of Philadelphia, 18.S6. p. 359. I 

 find upon a second examination of Prof. Emmons' original flgiire, that I did unintention- 

 ally criticise it too severely in the former article, p. 359. While far from accurate the 

 figure is not so misleading as I at first supposed. 



