OF THE MESOZOIC MAMMALIA. 
223 
upon the anterior and posterior slopes. The molars have perhaps a faint posterior, 
but no internal cingulum. The mandible is very stout with a lofty coronoid process. 
The condyle is probably midway between the angle and coronoid. There is a deep 
mylohyoid groove terminating beneath the molars. The symphysial surface is 
indistinct. 
MICEOCONODON, Osborn,' 1886. 
There is a wide diastema behind the 
Dentition. — I ? c pm^, m., or pm^, 
canine. The premolars are erect and sub- 
conical, with a faint posterior cingulum, 
and the third possesses a slight lateral 
groove in the fang. The molars have a 
shallow grooved division of the fang. 
There is a large median cusp and regular 
anterior and posterior cusps, in no case Figum a. Microconodm tenuirostris, outer face of 
1 rpi . T right mandibular ramus ; four times natural size. 
P e. ihere is a distinct cingulum Dotted contours of molars conjectural. Coll. Phila. 
posteriorly, which may represent the ex- Academy. Original. 
tension of an internal cingulum. The mandible is slender, the coronoid low; the 
condyle is probably low, and the angle is represented by a curvature of the lower 
border as in Peramns. 
JUEASSIC PERIOD. 
The Jurassic genera of the Second Group, like the triassic, possess a mylohyoid 
groove upon the inner surface of the mandibular ramus. They differ from the 
known triassic genera, first, in the complete division of the molar fangs; second, 
the premolars are not separated from the canine by a diastema, except in cases where 
they have evidently suffered numerical reduction. They are generally further dis- 
tinguished by the following characters: The incisors vary from four to three in 
number. The canines are invariably present, usually well developed and frequently 
bifanged. "With a few exceptions there are four premolars, well distinguished in 
pattern from the molars ; in other words the dentition is distinctly heterodont.' The 
molars vary from four to eight, generally exceeding the former figure, and present a 
variety of patterns of the cusped, but not of the tubercidar order. These features 
' “ Observations upon the Upper Triassic Mammals Dromotherium and Microconodon.” Proc. Phila. 
Acad. Jfat. Sc., 1886, p. 359. 
* Phascolotherium forms an apparent but not real exception, since the premolars have probably disap- 
peared. In Diplocynodon (Marsh, loc. cit. Plate X), the premolars when viewed upon the inner surface 
are very distinct from the molars, although the outer aspect of the series is uniform. Professor Marsh, 
however, gives the imperfect differentiation of the premolars and molars as one of the characters of the 
“ Pantotheria,” an order proposed for the reception of this group. 
