OF THE MESOZOIC MAMMALIA. 
201 
ACHYRODON, Plate IX, fig. 13 . 
As was fully recognized by Professor Owen, the molars of the minute Aclixjrodon 
are of the Amhlothermm type, but the genus is well distinguished by the acuteness 
of the cusps and the elevation of the premolars. The specimens consist of portions 
of four mandibular rami, the most perfect of which (No. 47,745) is represented in 
the figure. This is A. nanus, (Owen). The ramus is mutilated at the extremi- 
ties, but preserves two premolar casts followed by two premolars and eight mo- 
lars in situ. The posterior half of the symphysis is preserved, and from this the 
mylo-hyoid groove extends backwards to, and seems to perforate the border of the 
pterygoid fossa. Judging by the portion preserved, this fossa was divided into an 
upper and lower basin by a median ridge. Below the dental foramen is a lower ridge 
which extended into the angle. The coronoid border rises abruptly behind the last 
molar. The premolars are lofty and supported upon two fangs. The most anterior 
in this specimen is probably pm^, it has a very high crown, with a convex anterior 
and nearly vertical posterior slope, it has a pronounced cingulum, which slopes 
obliquely backwards and encircles the crown ; the last premolar has an equally high 
but more slender crown, rismg to double the height of ,• the cingulum is less decided 
but embraces the base of the cro\vn as in^jmg. The coronal patterns of molars are 
exactly similar ; there is no cingulum ; there is a high central cu.sp sloping forwards 
with a waving contour, and produced to a sharp point ; from its base rises freely the 
sharp and prominent anterior cusp and the low and comparatively blunt posterior 
cusp, which is, however, more elevated than in Amhlotlierium. The molars overlap 
each other. The first molar is relatively larger than in the allied genera ; the series 
increases gradually in size from to m^. The last molar lacks the posterior cusp. 
From the matrix impressions observed by Professor Owen in another specimen 
(PI. II, fig. 8 ; p. 40, Mes. Mamm.), it is probable that there were three or four 
incisors, a canine, four premolars and eight molars. 
LEPTOCLADUS. Plate IX, fig. 10 . 
This Purbeck genus is represented by a single fragment of a left mandibular 
ramus, with the outer surface exposed, and containing eight molars. Professor Owen 
(p. 53, PI. Ill, figs. 4, 4®. Mes. Mamm.), placed it as incertce sedis near Styhdon, 
remarking, however, that he considered this relationship somewhat doubtful. A very 
careful examination of this specimen, the results of which are expressed in the figure, 
shows that it is very remote from Stylodon; and, so far as known, represents an 
entirely unique type of dentition. The molars are seen upon the outer surface, and it 
is possible that a view of the inner surface would reveal the presence of internal cusps 
similar to those of the Peraspalax molars. The main molar cusp of each suggests a 
resemblance which is lessened by a close comparison of the molar pattern as a whole. 
When the .dentition of Leptocladus is carefully compared with that of each of the 
