50 Prof. Owen — On Castor and Trogontherium, 



indicate a proportionally large size of that tooth, repeating that 

 characteristic of the homotypal grinder in the lower jaw (Fig. 3, 

 p4). Only the first half of the crown of the last upper grinder is 

 in place, but the socket indicates a greater posterior prolongation of 

 this tooth than in Castor ; and a cast of the last three upper grinders 

 (m 1, m 2, m 3) of the Trogontherium found in lacustrine marl, in 

 the ' Department de I'Eure et Loire,' Fig. 2, (added to my Fig. 1, at m 

 3), shows the angular posterior production of m 3, which generically 

 differentiates TrogontJierium from Castor : it is associated, as we shall 

 see, with many corroborative modifications, if of minor value, in 

 other Trigontherian fossils. The molars (m 1 and m 2) in Mr. King's 

 specimen, from Mundesley, have a transverse section, or grinding 

 surface, more triangular than in Castor : the inner side of the tooth 

 is narrower and more rounded, not bilobed. The peripheral enamel 

 is continued along that side, uninterruptedly, becoming thinner as it 

 curves to the back part of the tooth. In Castor the inner enamel- 

 wall is bent into the substance of the tooth near the middle of the 

 inner surface, forming a fold, which extends nearly half-way across 

 toward the outer surface. 



The well-marked difference, in this respect, between the upper 

 grinders (m 1, m 2), of Castor and those of Trigontherium, is not due 

 to different degrees of attrition ; for the inner longitudinal groove 

 produced, in Castor, by the fold above-mentioned, is co-extensive in 

 that genus with the length of the enamelled crown. 



In TO 1 and in 2 of Trogontherium, there comes next, behind the 

 anterior enamel-wall, a narrow and long insular fold of enamel 

 curving across the crown nearly parallel to that wall, but slightly 

 receding from it, as the island extends from without inwards ; the 

 ends of the island touch, or nearly so, the peripheral wall of 

 enamel. There is no such narrow enamel-island curving from the 

 outer to the inner side of the front half of the molar in Castor ; it is 

 represented by an outer fold which joasses inward in front of and 

 beyond the . blind end of the inner fold above described, together 

 with that fold, in Castor. Behind the anterior enamel-island, in 

 Trogontherium, there is a second shorter island, sab-parallel there- 

 with, commencing from, or close to, the outer wall, and terminating 

 opposite the middle of the posterior border of the grinding surface. 

 In TO 1 and to 2 of Castor there are two folds of enamel penetrating 

 from the outer side, the anterior of which extends to near the inner 

 and hinder angle of the grinding surface. The last upper molar of 

 Trogontherium has the second transverse island parallel and co- 

 extensive with the first, with a third and a fourth shorter transverse 

 fold or island ; the grinding surface is triangular, with a curved or 

 convex anterior base ; the fore-and-aft extent equals that of the two 

 preceding grinders. In Castor the dimension of m 3 is less than 

 that of the contiguous molar, m 2, and its grinding surface, instead 

 of being more, is less complex. 



The lower molars, p 4,' to 1, to 2, of which the grinding surface 



' There seems to be more loss than gain in reversing the usual way, viz., from 

 before backward, of counting teeth ; and so I retain my method of considering the 



