OF DIDELPHYS VIRGINIANA. 



85 



obliquely upward and outward to connect the root of the small trochanter with the apex 

 of the trochanter major. The latter is remarkably large and strong ; its pointed apex rises 

 fully as high as the top of the globular head, from which latter it is separated by a deep 

 sulcus. The roughened surface of the trochanter to which muscles are attached, is of 

 great extent, broad and obliquely truncated above, below gradually narrowing and losing 

 itself in the shaft ; but the downward continuation of this trochanteric line may be traced 

 below the middle of the shaft, and chiefly gives the flattening of the posterior surface of 

 the bone, already mentioned. Anteriorly, the expansion of bone between the trochanter 

 and the neck is broad, smooth and nearly plane ; posteriorly, the corresponding surface is 

 interrupted by the inter-trochanteric ridge, and a remarkably large and deep "digital " 

 fossa. The latter is of a narrowly oval shape, and is excavated to an unusual depth — being 

 roofed over, as it were, by the backward expansion of the great trochanter. 



The hip joint is well adapted to extensive and varied movements of the thigh — rotation, 

 extension and abduction being specially favored, as usual with scansorial animals. The 

 amount of motion permitted seems not very much less than that enjoyed by the shoulder 

 joint, much of the mobility of the latter taking the place of the comparative stability with 

 which a hip joint is usually endowed. This quasi-quadrumanous animal requires that the 

 conditions of the proximal articulation of the fore and hind limbs should not be very dis- 

 similar. At the hip joint we find beginnings of a series of modifications in the hind limb ; 

 which, constantly augmenting, culminate in the production of a member that is as much an 

 arm as a leg. 



Tibia. — (Fig. 26.) The fibula does not properly enter into the formation of the knee 

 joint, the head of the tibia alone articulating directly with the femoral condyles. The 

 articular surfaces present nothing specially noteworthy ; the outer is de- 

 cidedly the larger, and is a little convex in all directions ; the inner pre- 

 sents a well-marked concave depression; the two are separated by an 

 irregular prominence. The outer one presents, besides its femoral facet, a 

 small, oval, transversely convex, smooth, articular surface, upon its outer 

 margin, which is received into a corresponding concave facet upon the inner 

 margin of the fibular head. This arrangement calls at once to mind the 

 peculiar radio-ulnar articulation ; and, in connection with certain peculiari- 

 ties in the head of the fibula, to be presently noticed, has been used as an 

 argument in favor of the homotypy of the ulna with the fibula and of the 

 radius and with the tibia. But the tibia does not in this case rotate upon 

 the fibula, but the reverse ; and it is immaterial which bone bears the con- 



cave, and which 



the convex, facet. The " lesser sigmoid flexure " of the 



ulna here finds expression in a prominence instead of a depression ; the 

 concave lateral facet of the fibula is adapted to this prominence, as the 

 convex lateral articulating surface of the radius fits the depression in the 

 side of the ulnar head. The anterior tibial tuberosity (the "olecranon of 

 the leg") is moderately prominent, and gradually subsides into the shaft of Fig. 26 

 the bone. The latter is much compressed laterally ; but its chief peculiar- 

 ity is found in its bending along the middle of its course toward the fibula, as well repre- 

 sented in the cut. A " crest " may be traced along the whole length of the bono ; but, 

 contrary to the usual rule, it is sharper and more elevated below than above, owing to 



Right Tibia 

 and Fibula. 



MEMOIRS BOST. 80C. NAT. HIST. VOL. II. 



22 



