the Recent and Fossil Beaver. 55 



The tibia of Trogontherium (PL III., Figs. 4, p, y, 5 and 6), besides 

 the superiority of size as belonging to a Eodent larger than the 

 European or Canadian beavers, differs in the much less depth of 

 the cavity (Fig. 6, r), impressing the back part of the proximal half 

 of the shaft : in the thicker and more obtuse inner (tibial) border 

 (s) of that concavity : in the stronger antero-posterior sigmoid flexure 

 of the entire bone : and the greater extent of the confluent distal 

 ends of tibia and fibula (w, y). The medullarterial canal (Fig. 6, v) 

 has a similar position and direction. The anterior ridge (Fig. 5, p) 

 is thicker and subsides sooner in Trogontherium : the outer concavity 

 (u), and inner flat or subconvex surface, resemble those in Castor. 

 The inner malleolus is longitudinally grooved as in Castor ; its back 

 part (z) is produced : the outer malleolus (y) projects a little above 

 the distal articular end of the fibula. 



Calcanemn, PI. III., Fig. 8. The calcaneum in both Castor fiber 

 and Castor canadensis, Fig. 9, is remarkable among rodents, for both 

 the considerable length and breadth combined, of the hind or fulcral 

 process, which, w^ith the articular part of the bone placed hori- 

 zontally for renewing and supporting a superincumbent vertical 

 tibia, is so inclined as to seem to be flattened vertically (depressed), 

 rather than from side to side (compressed).^ 



From the " Forest-bed " of Mundesley, Mr. Gunn obtained a 

 calcaneum (left) of this castorine type, (PL III, Fig. 8), equalling 

 that bone of a full-sized beaver in length, but exceeding it in 

 breadth. I assume it to belong to the Trogontherium. The articular 

 part of the bone presents two surfaces superiorly a, h, for the astra- 

 galus, and jDart of the larger anterior surface, c, remains on the 

 fossil for the cuboides. The outer and upper surface, a, is longer 

 and narrower in Trogontherium than in Castor ; the inner process, b, 

 is simply concave, not also convex, and flat posteriorly, as in Castor. 

 The fulcral process, d, is broader and thicker, relatively much 

 broader to its length, since in Trogontherium the bone is equally 

 divided between its articular and fulcral parts, whereas in Castor 

 the latter is the largest. In Castor the inner and lower border of 

 the fulcral part is almost straight in Trogontherium by the inclina- 

 tion of the tendinal canal to the under and fore part of the bone ; 

 the inner border of the fulcral process is somewhat convex. The 

 specific difference is obvious at a glance ; the degree of the dif- 

 ference brought out by detailed comparison suggests a generic 

 distinction concurring with that demonstrated by the dentition of 

 Trogontherium. 



The original or type specimen of Trogontherium has been described 

 and figured by Dr. C. Eouillier, in his " Jubilteum Fischeri," p. 35, 

 Tab. 5. It was discovered at Taganrog, Sea of Azoff. 



In the •' Quarantine Eavine," near Odessa, in a yellow argillaceous 

 deposit, beneath a thick stratum of " Calcaire d'Ossessa," the " Old 

 Caspian Deposit " of Murchison, various mammalian fossils- wore 



1 Catal. of Osteol. in Surgeon's College, 4to, No. 2193, p. 392. 

 - Eleplias, Eliiuoceros, Cervus, Equus, Hyteua, Ursus. 



