222 
ON THE STRUCTURE AND CLASSIFICATION 
B. SECOND GROUP. 
triassic period. 
While the American juraasic fauna is closely related to the British, the American 
.riassic fauna is widely separate from both, so far as we can judge from the scanty 
material which has been obtained from the North Carolina Beds. In the jurass.c 
forms of tall, countries the crowns of the molars are well distinguished Irom the 
fangs, and the latter are distinctly paired or multiple, ex- 
cept in cases where they are undergoing a secondary union 
{Kurtodan). In the two triassic genera known at present, 
the crowns are continuous with the fangs, and the only 
evidence that the fang is divided is a shallow median de- 
^ pression at the base which opens downwards. A similar 
Fit;. 8,— a. Inner aspect of division of the base of the croivn has been observed by 
Professor Cope, in Dtmetrodon, one of the Tlu,romorpl, 
ofthefang.i, Section of another j-eptiles of the Permian. This character is, therefore, of 
S^‘‘p.HpTivfty‘‘‘®(5‘,r'^Ue? great interest and importance, and necessitates the ordinal 
tion. orifrinal. • separation of these genera from those of the Jurassic period. 
ORDER PROTODONTA.— Osborn. 
Primitive lieterodont mammals in which the fangs of the molars and premolars 
were unpaired and not well distinguished from the crowns, the incipient division of 
tlie fang Iieing reprc.sented by a lateral groove on the base of the crown. 
DROMOTIIERIIDiE, Gill, 1874. 
'I'his family was proposed by GilP and adopted by Marsh% without definition, to 
inchule Dromotherium. It may now be defined to embrace also ^Rcroconodon, as 
follows: A wide diaeiema behind the canine. Premolars styloid and without a distinct 
heel. Molars with the main csisp and the lateral cusps in the same fore and aft line. 
DROMOTHERIUM, Emmons,^ 1857. 
Dentition — ijC^pm^m^. The incisors are caniniform, recurved and separate. 
1 he canine is large and recurved, followed by a wide diastema. The premolars are 
styloid and semi-procumbent with ungrooved fangs. The last premolar crown has a 
deep iK)sterior groove. The molars have imperfectly divided fangs. There is a lofty 
main cusp and irregularly disposed anterior and posterior cusps, sometimes multiple. 
By the AiUhor'!'"*®"''' ^dromotherium and Microconodon,” Proc. Am. Phil. Society, April, 1887. 
• “ Arrangement of the Families of Mammals, &c.,” Smithson. Misc. Coll., 1874, p. 27. 
• Am. Journ. Sc. and Arte, April, 1887. p. 344. 
• American Geology, Part VI, pp. 93 and 94. 1867. 
