FOSSIL MAMMALIA. 91 



condyles, as in the Megatherium. In fig. 1. PI. XXV. which gives a front view 

 of the left humerus, the broad internal condyle, with its extremity broken off, is 

 seen projecting to the left hand ; both in this figure and in fig. 2. in which the 

 internal side of the humerus is turned towards the observer, the wide groove, with 

 its two osseous boundaries, is shewn, which plainly indicates that the left condyle 

 was perforated for the direct passage of the artery or median nerve, or of both, to 

 the fore-arm. The groove for the musculo-spiral nerve on the outer side of the 

 humerus is over-arched at its upper part by a strong obtuse process ; which is 

 comparatively less developed in the Megatherium. The trochlear or inferior arti- 

 cular surface of the humerus presents, as in the Megatherium, two well-marked 

 convexities, with an intervening concavity : this indication of the rotatory power 

 of the fore-leg is confirmed by the form of the head of the radius. 



In PI. XXV. fig. 4. a view is given of this articular surface : it presents the 

 form of a subcircular gentle concavity, which plays upon the outer convexity of 

 the humeral articular surface : immediately below the upper concavity the radius 

 presents a lateral smooth convex surface, which rotates upon a small concavity 

 on the ulna, analogous to the ' lesser semilunar,' in human anatomy, in which 

 the mechanism for rotation, so far as the upper joint of the radius is concerned, 

 is not more elaborately wrought out than in the present extinct edentate quadru- 

 ped. The radius expands as it proceeds to the elbow-joint, where it attains a 

 breadth indicative of the great power and size of the unguiculate paw, of which 

 it may be called the stem, and to the movements of which it served as the pivot. 



All the bones of the fore-limb just described — the scapula, the humerus, and 

 the radius, — indicate by the bold features and projections of the muscular ridges 

 and tubercles the prodigious force which was concentrated upon the actions of 

 the fore-paw, and the ulna, in its broad and high olecranon (of which a side-view 

 is given in fig. 2. PI. XXV.) gives corresponding evidence. The great semilunar 

 concavity is traversed by a sub-median smooth ridge, which plays upon the in- 

 terspace of the two humeral convexities. The body of the bone is subcompressed, 

 straight, and diminishes in size as it approaches the carpal joint : the immediate 

 articulating surfaces are wanting in both the radius and ulna, the epiphysial 

 distal extremities having become detached from their respective diaphyses. 



Of the terminal segment of the locomotive extremities, the only evidence 

 among the remains of the skeleton of the Scelidothere is the ungueal phalanx 

 figured at PI. XXVII. 3, 4, and 5 ; but as it is uncertain whether it belong to 

 the fore or hind-foot, it will be described after the other bones of the extremities 

 have been noticed. 



Of these bones the femur is the most remarkable, both for its great 

 proportional size, and its extreme breadth, as compared with its length or thick- 



N 2 



