38 



80ricinum (\h., fig. 1). The pterygoid fossa is bounded by a well-marked inwardly pro- 

 duced ridge, which has declined to the lower border of the ramus at the hind fracture. 

 The dental canal (ib., fig. 5 a, d) begins at the fore part of the fossa, below that border of 

 the coronoid. The mylohyoid groove (g) begins in advance of the lower bounding 

 ridge of the inner fossa, and, if it lodged a nerve of that name, this must have per- 

 forated the ridge anterior to the entry of the mandibular nerve-trunk, in order to 

 emerge upon the groove. The groove is well-defined, linear, and extends at first 

 straight forward, then very slightly bends up to the back part of the symphysis (r). 

 This is narrow, long, and continues the feeble convexity of the under border of the ramus, 

 without any abrupt upbending from that contour. Below the last three molars the inner 

 alveolar plate is subconvex vertically to the groove. Beneath the anterior five molars 

 the bone is flatter : in advance of the premolars the inner surface bends inward as it sinks 

 to the symphysis. 



Achyrodon differs from Jtublothcrium in the greater relative length of the anterior cone 

 of the molars, which projects over the hind cusp of the antecedent tooth, exemplifying the 

 closer, and as it appears interlocked, array of the molars in the present genus. Admit- 

 ting that one molar may be wanting in the type of Amblotherium (PI. II, fig. 1), the seven 

 molars then would occupy the same relative extent as do the eight molars in Achyrodon. 

 They are, nevertheless, in other respects so closely alike as to have led me long to hesitate, 

 and repeatedly pass under comparative review all the Purbeck specimens showing this 

 general slender-coned type of molar, before concluding to indicate the differences above 

 defined by a generic name. 



The specimen (PI. II, fig. G, nat. size, fig. 6 a, magn. three diam., in tint) might seem 

 to be the left ramus of the same mandible as the right one last described, but that it 

 appears more slender. Like that, it is mutilated at both ends, but in a less degree, 

 retaining the two anterior premolars, and rather more of the reflected lower border of 

 the ' ascending ramus.' There are five molars (m 1—5) and sockets for three, if not 

 four, behind the teeth in place. The crown of the best preserved teeth show the base 

 of the long anterior cone and the smaller and lower posterior basal cusp, united by a low 

 oblique smooth rising ; external to which, or from which, ascends the principal cone, 

 chiefly supported by the outer side of the coronal base. The molars are close-set ; and, 

 indeed, their bases seem, as in the right ramus (fig. 5), obliquely to overlap each other. 



The apex of the main cone is broken in all, or more worn than in fig. 5. The last 

 premolar (p 4) shows the usual simple type of one chief subcompressed pointed cone; 

 which, though broken, evidently had the characteristic superiority of height over the 

 succeeding true molar (m 1). It is implanted, as in fig. 5, by two roots. 



The ramus is slender, almost straight. The slight curve of the lower border, and the 

 indications of the ascending ramus (a and c) repeat the characters in the mandible (fig. 5). 

 The deep pterygoid fossa leads to the entry of the dental canal (d) which is below the 



