52 Prof. Owen — On Castor and Trogonthermm, 



from Baoton (fig. 71, Brit. Foss. Mamm.) ; and in 1845 I could only 

 remark, in regard to w 3, tliat its socket shows it to have had a 

 larger antero-posterior diameter than the antecedent grinder, corre- 

 sponding with the character of its homotype above, both molars 

 showing proportions the reverse of those in the genus Castor. In my 

 last visit to Norwich, I found in the well-known collection of Eobert 

 Fitch, Esq., F.G.S., which was opened to my inspection with that 

 gentleman's usual liberality, a portion of the right ramus of the 

 mandible of Trogontherium, from the Forest-bed at Mundesley, having 

 the series of four grinders complete. The specimen was also from a 

 younger individual than those I had previously seen, showing rather 

 larger dimensions of m 1 and to 2, which slightly con- 

 tract toward the roots, and afi"ording a more perfect 

 condition of the enamel character. In p 4, the small 

 anterior island of Fig. 3 retains (b) its character as a 

 short fold from the outer part of the anterior end 

 of the tooth. The beginning of the outer submedian 

 fold, a, is rather wider, the continuation of the two 

 inner folds, h, c, with enamel- wall is more distinct. 

 This beginning of the folds a, b, e, is also manifest 

 in m 1, and to 2. In ra 3, the two minor folds, h c, 

 diverge from each other as they penetrate the tooth 

 more than in the other grinders ; the outer fold a 

 inclines more obliquely backwards ; there is an indi- 

 cation of a very short bend of the enamel-wall at the 

 produced and rather contracted hind-end of to 3. In 

 of the Molar Series, all the teeth the anterior long and narrow enamel- 

 l'^\iTro^n7he- inland is shown ; and in to 2 and to 3 are indications 

 r»Mm,nat.size. (Mr. of its being an infolding from the inner enamel- wall. 

 s specimen.} j^ Castor the last grinder of the mandible, like that 

 of the maxilla, is smaller than the rest, which, with the difference 

 of pattern of the grinding surface of all the molars, gives ground 

 for generic distinction of the great Castorine rodent of the Forest-' 

 bed, as good as for any accepted genus of the order. 



The outer part of the mandible in Trogontherium is much more 

 prominent or convex below the small crotaphyte fossa than in Castor. 

 The rough surface for insertion of masseter extends to beneath the 

 anterior grinder, jp 4; in Castor it does not advance beyond the 

 interspace between p 4 and to 1. 



A distal epiphysis of a right humerus, obtained by Mr. Gunn from 

 the Forest-bed of Mundesley, indicates a larger proportional size of 

 the fore-limb in TrogontJiermm than in. Castor. The radial convexity 

 of the trochlea has more fore-and-aft extent in Trogontherium ; its 

 contour is nearly that of an equilateral triangle, with the sides 

 rather convex : in Castor the transverse diameter of the radial con- 

 vexity much exceeds the antero-posterior one. The ulnar channel is 

 much deeper in Trogontherium than in Castor : the posterior or 

 olecromal part of the pulley is relatively wider and shallower in 

 Trogontheritim. 

 In an old British beaver {Castor fiber), from a Cambridgeshire 



