or THE MESOZOIC MAMMALIA. 
20.1 
cusps. A fomth specimen (fig. 13, Mes. Mamm.) may, however, be safely referred to 
Peramm, but, unfortunately, it adds little to our knowledge of the dentition. The 
jaw of Peramus is quite unique. The coronoid process is very high and tapers as it 
ascends ; its summit is fractured, but a faint cast indicates a rounded contour ; its 
anterior border descends in a ridge which extends along the outer face of the ramus 
beneath the molars. The condyle is on the molar level and terminates a ridge. 
Below this, the border descends into a triangular process which, according to Profes- 
sor Owen, is sharply inflected and thus represents the angle. This observation 
cannot now be confirmed as this process has been broken away. The ramus tapers 
anteriorly but less so than in Stylodon. There is a foramen below the first and 
second premolar. The fourth specimen seen upon the inner surface shows a mylo- 
hyoid groove. 
In the matrix of the anterior portion of the type specimen there can be distin- 
guished a faint cast of the canine ; behind this are traces of nine teeth, six of which 
are preniolars ; the paucity of molars is a unique feature not elsewhere observed 
among the mesozoic mammalia of this type. The sixth tooth is counted among the 
molars by Professor Owen, but it has a distinctively premolar pattern and lacks the 
elevated anterior cusp of the true molars. The premolars increase from first to last ; 
they all show the usual recurved crowns and low posterior heel, while have 
anterior basal cusps indicating the presence of a strong internal cingulum. The 
pattern of the three molars is substantially the same, being the typical tooth of 
the series. It has a high central cusp with a strongly convex outer surface ; from 
the base of its posterior slope rises a prominent heel, while from the upper internal 
surface of its anterior slope rises a small cusp; this cusp, although rotated inwards, is 
accessory to the main cusp and is not an independent inner coronal cusp, such as that 
in the Peraspalax molar, as Professor Owen’s description (p. 41) and figure 10 c 
(PI. II) would indicate; a minute cusp at the base of the anterior slope probably points 
to an internal cingulum, and we may conjecture that the inner surface of this crown 
was broad and shelf-like. M and are slightly smaller than m^, and the anterior 
basal cusp, if present, is faint. 
Since the above was written, much evidence has come to hand that the number 
of premolars in all the mesozoic mammalia never exceeded four. This rule is so 
universal that it seems unlikely that Peramus should form the single exception. 
It is therefore possible that Avhen the inner faces of the teeth are discovered, those 
determined as pm^ and pm^ will prove to be m^ and and the formula will stand 
pm 4, m 5. 
SPALACOTHERIUM. Plate VIII, fig. 7. 
This genus, from the Purbeck, is represented ‘by numerous mandibular fragments 
with well-preserved teeth. The two here chosen and combined for illustration as 
types, are: first, the left ramus (No. 47,750, Brit. Mus,), figm-ed by Professor Owen 
