the Recent and Fossil Beaver. 



51 



is figured of the natural size in my " Britisb. Fossil Mammals," 

 p. 189, fig. 73, are from an aged individual, with crowns worn 

 down to near the base. As both m 1 and m 2 were in place and use 

 before p 4, they show the efi'eots of attrition in a greater degree. 



In this portion of lower jaw obtained by Mr. King from the 

 Forest-bed at Mundesley, and the remarkable similarity of which 

 to the figured specimen from Cromer ^ is noted by my accomplished 

 and much lamented friend, the degree of wear is less, and the cha- 

 racter of the grinding surface of m 1 and m 2 is better shown. 



The outer side of the crown is not indented, ^_ , 



so as to become bilobed, as in Castor ; the fold 

 of enamel which extends from the outer side 

 half-way across, and corresponds with the wider 

 fold dividing the lobes in Castor, begins more 

 abruptly, aiad as a closer fold, of the outer enamel- 

 wall ; and, as the fold or in-doubling is not con- 

 tinued so low down the outside of the crown in 

 Trogontlieriiim, the fold is sooner separated from 

 the outer enamel-wall, and appears as a narrow, 

 slightly bent island, curving from the outer, 

 obliquely backward, half-way toward the inner 

 side of the crown. Anterior to this fold are 

 two narrow enamel-islands : one begins close to 

 the inner enamel- wall, or as an infolding of that 

 wall, and runs pai'allel with the outer fold or 

 island, more than half way to the outer side. In 

 advance of this is a narrow island of enamel near portions of Left ramus 

 to the anterior enamel -wall, with the ends Lower jaw, (Trogonthe- 



• -<• , I o IT , -\ • IT n rium Cuveiri,) nat. size. 



equidistant Irom the outer and inner borders of Rev. s. w. King's speoi- 

 the grinding surface ; this island is nearly oblite- ^^'^• 

 rated in m 1 ; as it was wholly so in the Cromer specimen (fig. 73, op. 

 cit.) Behind the first-mentioned island or fold, from the outer surface, 

 is one from the inner surface, extending parallel with the posterior 

 enamel-wall rather more than half-way across the tooth ; this last 

 coronet fold is quite insulated and shortened in m 2 of the Cromer 

 specimen, and has disappeared from m 1. 



This anterior grinder, 4, p shows an insular remnant of an enamel 

 fold anterior to the four long islands or folds in fig. 73, Brit. Foss. 

 Mamm., and a rather more produced and narrower anterior termi- 

 nation of the crown than in the specimen of the older TrogontJiere 

 from Cromer. 



It is remarkable that the hindmost grinder should be wanting in 

 the five examples of mandible, including the more perfect specimen 



hindmost of the pre-molars, like the bindmost of the true molars, as the last of its 

 series, -which in Biphijodonts is the fourth. The order in which both kinds of teeth 

 follow one after another, viz., from before backward (if there be an interval in 

 appearance), also weighs with me in rejecting Rntimeyer's innovation of countiuo- 

 the hindmost and last appearing milk-molar and pre-molar, as the "first" of their 

 respective series. — " Beitrage zur Kenntniss der fossilen Pferde," in " Verhand- 

 lungen der Natur-forsch. Gesellsch. in Basel," Bd. iii. Ht. 4, 1863. 

 1 Brit. Foss. Mamm. p. 186, fig. 72. 



