54 Prof. Owen — On Castor and Trogontherium, 



is as mucli developed as in Castor, "but is more triangular in shape, 

 less thickly and obtusely terminated than in Castor. The chief dif- 

 ference, however, is in the third trochanter, which in Trogontherium, 

 is represented by a ridge (ib. e, d), continued from the outer part of 

 the base of the trochanter major downward and slightly backward, 

 subsiding before attaining the mid-length of the bone, a little behind 

 the outer border of the shaft at that part. In Castor the corres- 

 ponding ridge is thinner, sharper, and extends directly outward, 

 thickening and projecting as an obtuse anterioiiy bent process 

 from the mid-half of the outer side of the femur, a ridge is con- 

 tinued down from the base of the process to the ectocondyloid 

 tuberosity. In Trogontherium there is a channel (Fig. 1, e) along 

 the fore part of the ridge d, that causes its free border to curve 

 forward, though in a less degree than in the process of the beaver's 

 thigh-bone. There is also a depression indicative of muscular in- 

 sertion at the back of the third-trochanterian ridge (Fig. 2, /) in 

 Trogontherium. After the subsidence of this ridge, the shaft of the 

 femur is free from ridges, etc., for the extent of nearly an inch, 

 yielding from its compression, a transverse irregularly oval section. 

 No part of the shaft of the beaver's thigh-bone is in this condition; 

 like the mole's humerus it is invaded from end to end by outstand- 

 ing processes or ridges. 



The outer border of the distal third of the femoral shaft of Trogon- 

 therium assumes by its expansion and posterior excavation (Fig. 2, k) 

 the character of a ridge, but much thicker and more obtusely termi- 

 nated than in Castor, in which the well-defined posterior "muscular" 

 depression is wanting. The inner side of the distal third of the 

 femoral shaft retains the transverse convexity to the entocondyloid 

 tuberosity (Fig. 1, i.); in Castor it is produced and thinned off to an 

 edge ; the tuberosity is alike in both, but a little more defined from 

 the shaft in Trogontlierium. Above the rotular canal there is a large 

 smooth depression (Fig. 1, Tc.) in Trogontherium, scarcely a trace of 

 ■which exists in Castor; the rotular canal (Fig. 1,1.) has the same 

 breadth and outer inclination as it descends, in both. In bo th, also, 

 its articular or synovial surface is detached from that of either femoral 

 condyle, thoiigh nearest to the outer one, m. As the distal end of 

 the femur is relatively less expanded than in Castor, the rotular 

 canal is broader in the like relation in Trogontherium. The posterior 

 intercondyloid channel (Fig. 2, o) is deeper and narrower in 

 Trogon therium. 



I am not cognisant of such differences in femoral structure in two 

 species of any genus of Eodent, now accepted in Mammalogy ; and 

 regard those above specified as concurring, with the dental differences, 

 in establishing the genus Trogontherium in the Castorine group. I 

 also infer from the femoral modifications, that the Trogontherioe was 

 less aquatic and a swifter mover upon land than the Beaver. This 

 inference has some support from the characters of the tibia ; for, if 

 the present femur have attained or nearly attained its full-length, 

 "wanting only consolidation or complete confluence of its distal epi- 

 physis, it is shorter in proportion to the tibia than in Castor. 



