OF THE MESOZOIC MAMMALIA. 
19& 
is that the fourth molar does not come into place at all ; in several of these specimens 
the jaw is to all appearance fully mature. It may be that in some species is 
more letaided than in others. While therefore the evidence as a whole favors the 
above hypothesis, there remains the possibility that m , is wanting in Triconodon and 
characterizes the allied genus Triacantliodon. In such case the T. mordax, Willett, 
should be added to the latter genus. 
AMBLOTHERIUM, Plate IX, fig. 11. 
W hen the genera Amhlotherium, Achyrodon and Phascolesies are placed side by 
side, as upon Plate IX, a striking general resemblance is at once observed. This is 
extended when the dentition is carefully examined. The molars especially are 
essentially similar in form and number and distantly recall those of Amphitherium. 
Amhlotherium is the best preserved and most representative member of this little 
group. It is known from two well-preserved mandibular rami which belong 
to different species and enable us to determine all the characters of the lower jaw' as 
seen upon the internal surface. The figure is taken ixom A. soricinum^, (No. 47,752.). 
The condyle is raised above the molar level, upon a broad condylar process. 
The angle is slender and slightly inflected at the tip, as in Amphitylus ; a ridge 
extends from this to unite with the thick anterior border of the coronoid 
and thus bounds the deep pterygoid fossa. In the angle thus formed is the dental 
foramen from which the mylohyoid groove reaches forward to the symphysis. The 
inferior border of the ramus has a double curvature and ascends anteriorly, tapering 
to the symphysis. The symphysis is long and narrow and the incisor border is well 
raised, while that of the premolars is depressed. There are four incisors, a canine, four 
premolars, six molars wfith the impression of a seventh in sihi. The whole dentition 
is rather 5veak, the incisor-canine series especially so. The incisors are widely sep- 
arated, with slightly expanded sub-recurved crowns set upon slender fangs; they 
decrea.se in -size from % to the latter being very small. The canine is a slender, 
recurved tooth, with a single contracted fang. The premolars increase rapidly in size 
from before backwards ; pm-^ is a minute tooth of the premolar pattern most prevalent 
in the mesozoic mammals, i. e., a recurved crown, bifanged, with a low posterior heel 
and a faint internal cingulum ; in pm 3 the heel is more prominent; prn^ has a high 
crown and less distinct heel. A characteristic feature of the molars is the absence of 
an internal cingulum. They increase in size from nii to m^ and then decrease ; they 
are all bifanged and have a uniform pattern. The third molar is typical of the series; 
the croivn supports a high central and two lateral cusps ; the central cusp is high, 
pointed and slightly recurved ; the anterior cusp is directed obliquely forwards and 
upwards, while the posterior has rather the character of a basal heel, which is over- 
* There is little doubt that some of the specimens referred by Prof Owen to Amhlotherium, belong to Stylodon 
or the Stylodontidx, with a styloid external cusp. 
