42 



of the base of the main cone indicates an accessory part, which represents the antero- 

 internal cusp in PeraspaleUB and Didclphys. The three molars showing the above structure 

 progressively, but slightly, decrease in size as they recede in position. 



The main cone in the foremost of the four teeth in place {in 4) has its outer surface 

 less bulging or convex across, and its acute apex is on a lower level than that of the next 

 molar {m 5). The anterior cusp is better marked; but the posterior basal cusp is still the 

 largest, though less than in the succeeding molars. The antecedent alveoli sufficiently 

 show the multidentate character of the fossil ; but it would seem that the premolar shape 

 and degree of coronal simplicity prevailed over a greater proportion of the molar series than 

 in Peraspalax. 



The condyle {b) projects backward on the level of the dental series. The angle of 

 the jaw (a) bends downward and backward, as a short triangular process, in advance of 

 the condyle and from a lower level. Working out the matrix with careful avoidance of 

 disturbance or breakage of so rare a feature in the present series of small jaws, I got the 

 evidence that the angular process was slightly inflected, as shown in the view from below, 

 in fig. 10 b, a. The fractured surface at the fore part of this process was probably due to 

 the original exposure of the fossil. The dotted line in fig. 10 a indicates the proportion 

 of the process which I suppose may then have been broken off. The crotaphyte depression 

 is deep and extensive from behind forward ; its fore and under boundary ridges are well 

 marked. The ridge, continued from its anterior margin, curves forward midway between 

 the molar alveoli and the lower border of the ramus, and, blending with the lower ridge, 

 subsides in advance of the four teeth in place. 



Peramus tenuirostris differs from Spalacotherium trieuspidens in the non-constriction 

 of the mandible behind the molar series ; in the sharper angle at which the external 

 crotaphyte boundary-ridges meet anteriorly ; in the lower position of the condyle (5) ; as 

 well as in the deflected angle (compare fig. 10 with fig. 38 in PI. I). Peramus also differs 

 in the two last-defined characters from Amblotherium, and notably from AmbJotherium 

 mustelu/a, in the greater slope of the fore margin of the coronoid process. 



Sufficient of the jaw of Achyrodon is preserved, in figs. G and G a, PI. II, to show a 

 difference of contour of the lower border of the ramus, but the generic distinction is more 

 decidedly manifested in the forms and proportions of the molar teeth. The contrast which 

 the mandible of Peramus presents with that of Amphitherium and Phascolotherium will be 

 seen by a glance at figs. 23 and 20 in PI. I. 



The next example of Peramus tenuirostris is the fore end of the left mandibular ramus 

 wanting in the preceding specimen. It is outlined cf the natural size, PI. II, fig. 11, and 

 given in tint at 11 a, inagn. 3 diam. It includes the bases of three incisors, a canine, 

 and five premolars. 



The jaw tapers in a singular degree to its fore extremity, the line of the incisive alveoli 

 coming down to meet that of the symphysis, while this is nearly horizontal or slightly 



