OF THE MESOZOIC MAMMALIA. 
239 ' 
LEPTOCLADUS, Owen,i 1871. 
Dentition pm m The lower premolars have a recurved main cusp with a 
faint cingulum upon the anterior slope and a posterior basal heel. The molars have 
a faint external cingulum and two fangs. The main cusp is elevated, and recurved. 
The second cusp forms a wide, low posterior heel, which is barely distinct from the 
cingulum, extending around the outer face of the crown. 
III. THE ORIGIN AND SUCCESSION OF THE TEETH IN THE MESOZOIC MAM- 
MALIA. 
The tooth forms have been so fully discussed in the preceding section, in their 
bearing upon classification, that it remains here first to briefly recapitulate the differ- 
ent types ; second, to point out their probable origin and succession. 
B. — Second Group. 
Incisors and Canines. — In its entire dentition Dromotherium is separated from- 
the whole Jurassic group. The incisors are caniniform and widely separate, increasing 
rapidly from the median incisor to the canine. In the Stonesfield Slate genera, the 
earliest of the English Jurassic, Amphitijlus and PhascolotJierium, the incisors differ 
widely ; they are styloid and separate, while in the later Triconodontidm they become 
close set, recurved and prehensile. Other degrees of specialization have been pointed 
out in the Sti/Iacodontidce. The frequency of the bifanged canine, in all the sub- 
groups, reverts to an earlier, homodont condition in which the canine was less 
differentiated from the premolars. In Pliascolestes the median incisor also has 
a grooved fang. In Amphitijlus the canine is apparently premolariform. In the 
Stonesfield Slate genera, the canine is usually small, and resembles a large premo- 
lar, but it assumes large proportions in the upper Jurassic genera. 
Premolars and Molars. — The premolars of Dromotherium are very unique. 
They are tall and styloid and single fanged; the last premolar has a vertical groove 
upon the posterior face. In 2Iicroconodon, which belongs to a somewhat more recent 
type, the premolars have a faint posterior heel and the last shows the trace of a double 
fang. In all the Jurassic genera the premolars, where fully functional, are bifanged,, 
and possess a convex anterior face and concave posterior slope terminating frequently 
in a heel. As in the molars, the cingulum plays an important part in connection 
with the basal cusps. It is present upon the internal face of the premolars of all the 
1 “ Mesozoic Mammalia,” p. 53. Leptoeladus dubius. 
