PURBECK FORMATIONS. 38 



§ VIII. Genus — Peralestes,^ Owen. 



The present genus, like Amblofheriiim, differs from Spalacofherium by a more marked 

 distinction between the preinoLirs and molars ; the former being necessarily characterised, 

 through the want of developmental evidence of these long-since extinct Mammals, by shape 

 and relative size of crown. 



In Spalacotherium, as in Amphitherium and Phascolotherium, it is difficult to draw the 

 line between the members of the numerous molar series, as may be appreciated by reference 

 to the subjects of figs. 23, 25, 26, 33, PI. I. Here, however, the superior length of crown 

 serves to determine the last and penultimate premolars by contrast with the smallness of 

 the tooth by which the molar division of the ' cheek-series' begins. 



Species 1. — Peralestes longirostris, Owen. Plate II, figs. 3, 3 a, B, c. 



Of the molary series of teeth evidence is given in the present species by instructive 

 portions of the dentition of both upper and lower jaws. 



PI. II, fig. 3, represents of the natural size, in outline, a portion of the right upper 

 jaw, which at a is figured, in tint, magnified 3 diam. ; whilst at b, the grinding surface of 

 the preserved upper molars is represented on the same scale. 



This specimen includes the external alveolar wall of the maxillary with the last pre- 

 molar and six following true molars in sitii. As much of the upper jaw as equals in length 

 the series of preserved teeth extends in advance of them : it is a fragmentary tract, but 

 shows part of the bony palate and a few alveoli. A vacuity near the fore end of the 

 palate, and a corresponding fissure on the outer wall of the jaw, may indicate the hind 

 limits of a premaxillary ; but this portion of bone is not entire anteriorly. 



The interest and instruction afforded by this specimen lie in the demonstration given 

 of the character of the crowns of the upper molar teeth, and especially of the configura- 

 tion of their outer side. 



In a direct side view of the exposed part of the specimen the outer wall or division 

 of the crown shows but little vertical extent compared with the fore-and-aft, or with the 

 transverse, diameter ; but the inner half is developed into a longer cone. The front half 

 of the outer wall presents the form of a smooth subhemispheric protuberance («), from 

 which the hind half extends backward at a lower level ; its outline being broken by a few 

 slight notches dividing it into two or three minute tubercles {b). The larger antero- 

 external tubercle {a) contracts to a low cusp (sharper in m 4 and 5 than in m 3), from 

 which the grinding surface curves outward to the loftier or longer and more acutely 

 terminated cone of the antero-internal lobe (c). The post-external lobe or tract {h) con- 

 tracts as it extends inward, and ends there in a low cusp (ib , fig. 3 b, d, d) at the hind 

 and inner part of the base of the long antero-internal lobe (c). The horizontal section of 



' 7ri/pa, pouch ; \i)aTr\s, robber. 



5 



