420 PLAGIAULAX. 



anterior third of the ramus, the fracture having passed 

 vertically through the middle of the last premolar. The 

 incisor is broken off near the base of the exserted portion ; 

 the two anterior premolars are quite entire, and show well 

 the diagonal grooving on both sides. The alveolar sheath of 

 the incisor is entire all round, and its upper edge presents an 

 exceedingly abbreviated diasteme. The symphysial portion 

 is very short, directed upwards, massive, and obtuse. The 

 disc of the harmonia or junction between the two mandibular 

 pieces is distinctly denned (fig. 12, b). It is of comparatively 

 small antero-posterior extent, of a broad elliptical or some- 

 what reniform outline, with the sinus directed backwards, 

 and the long axis of the disc with but slight deviation from 

 the vertical. The surface is perfectly smooth and without 

 indentation, as is commonly seen in Hypsiprymnus and other 

 herbivorous marsupials; while in the carnivorous and in- 

 sectivorous genera of the same subclass, the symphysial 

 harmonia is narrower and more elongated, with more or less 

 inequality of surface for the reciprocal firm apposition of the 

 united pieces. It is of some importance to take due notice 

 of this character, however trivial it may appear, as every 

 point is of value that can assist us in determining the affini- 

 ties of this remarkable fossil genus. On the outer surface of 

 this fragment the mentary foramen (fig. 11, c) is seen under 

 the middle of the diasteme. It is round, well defined, and 

 of good size. This instructive fragment furnishes direct 

 evidence upon another point of importance, namely, that there 

 was but one large incisor on either side of the lower jaw. 



It would have been of great interest and importance to 

 have ascertained the character of the outer surface of the 

 ascending ramus in Plag. Becklesii ; that is to say, whether, 

 in harmony with the marsupial indications as here interpreted, 

 it presents a depression bounded by a raised ridge sweeping 

 round on the lower side from the condyle to join on with a 

 corresponding ridge descending along the anterior margin 

 of the coronoid ; and whether the depression terminated in a 

 trumpet-shaped excavation of the horizontal ramus, common 

 to it and the dentary canal, as occurs in Hypsiprymnus. But 

 the fossil is so fragile, that I have not attempted to detach it 

 for fear of injuring what it now exhibits. We may expect, 

 however, if the excavations are continued at Purbeck with 

 the same zeal with which they have hitherto been conducted, 

 that abundant materials will be acquired for clearing up this 

 single unascertained point connected with the characters of 

 the lower jaw of Plag. BecMesii. From the direction which 

 the fracture has invariably taken along the line of least re- 

 sistance, in two fragments on reversed slabs, whenever a 



