PURBECK FORMATIONS. 91 



The premolar (fig. 11, jy 4) is reduced in size, and begins to assume the triturating 

 structure and breadth- In the true molars the four lobes have each the shape of a three- 

 sided pyramid, with the inner side of the inner pair of lobes undulated or indented in a 

 manner recaUing the ruminant type of grinders. 



The lower incisors in Phascolarctos and Hypsiprymnus show, as in Rodents, an abraded 

 surface (fig. 11, i) indicative of habitual and long continued gnawing actions. The man- 

 dible, by its lofty condyle (ib. c), its short, narrow, recurved coronoid (5), and the low 

 relative position in which the inflected angle («) is produced, concurs with the herbivorous 

 type of dentition in all Carpophaga and Poephaya. 



In Halmaturus the mandibular incisor is procumbent ; its long pointed crown is 

 depressed, rather expanded or spatulate, with an outer trenchant edge, the inner side more 

 or less flattened by pressure against its fellow, and the upper side showing, for some 

 extent behind the point, the plane of attrition produced by action against the three opposing 

 incisors of the premaxillary. The trenchant premolar, though relatively larger and more 

 compressed than in Macropus, especially in such large extinct Wallabees as Halmaturus 

 Atlas, forms but a small part, one ninth, of the dental series. It is followed by four 

 molars with large cubical bilophodont crowns, with superadded longitudinal and trans- 

 verse ridges. Each molar averages three fourths of the fore-and-aft extent of the 

 trenchant premolar.^ 



In Hypsiprymnus the long pointed incisor (fig. 14, i) is likewise procumbent, but is 

 trihedral. The outer side is convex across, the inner and the upper sides are narrower 

 and are flat, but are not divided by so sharp an angle as that which bounds the outer 

 facet. The upper surface of the incisor shows a plane of attrition from action upon 

 the upper incisors for one third of the extent from its apex, and the enamel is wanting 

 on this surface. The trenchant premolar (ib./; 4) has alow crown with a straight cutting 

 subserrate edge. The outer and inner sides meet at a much less acute angle than in 

 Playiaulax. The inner side shows five or six vertical ridges, the first and last being 

 broad. The outer side is worn smooth in old individuals. This tooth forms one seventh 

 of the longitudinal extent of the entire dental series. It is succeeded by four molars 

 [m 1 — 4) with large cubical massive crowns, the grinding surface of which, characterised as 

 described by Waterhouse,^ become, by age, worn down to a flattened, more or less irregular, 

 triturating surface, exemphfying the habitual horizontal, rotatory, or alternate crushing 

 action of the mandible upon the upper jaw. 



There is but little variation in the rise from horizontality of the lower incisor in the 

 known Potoroos (fig. 14) and Bettongs (fig. 13) j in none is the upper contour of the 

 exserted part of the tooth ii) raised above the parallel of the alveolar border of the lower 

 molars. 



In a skull of the Brush-tailed Bettong {Bettonyia penicillafa, fig. 13) now before me, 

 the scalpriform character of abrasion of the upper surface of the pointed ends of the 



' 'Odontography,' pi. 101, fig. 3. 2 Qp. cit., p. 194. 



