OF DIDELPHYS VIRGINIANA. 87 



existintr between the humerus and radius. Whether as cause or effect of the shallow 

 anterior intercondylar groove of the femur, the patella is deficient, as in many other 

 marsupials that have similarly constructed femoral condyles. In most instances, there is 

 barely an indication of a patella in a slight, apparently cartilaginous, thickening of the 

 extensor tendon ; in some there is absolutely no trace of such a bone ; in old subjects, one 

 or several small specks of ossification, seeming to the touch like gritty particles, may be 

 demonstrated. In this animal, at least, there is no " olecranon of the leg," detached or 

 otherwise, unless such process is represented by the " tuberosity of the tibia." The most 

 noticeable feature of the knee joint is found in the presence of the fabella, developed in 

 the outer lateral ligaments. This is an osseous nodule of an oval shape, with an enlarged 

 base, surmounting the most external and posterior corner of the head of the fibula. It 

 has a real articulation with the latter, a smooth, plane facet, subcircular in outline, being 

 plainly shown upon the opposed surface of each bone. The presence of this ossicle may 

 relate in some way to the rotatory movements of the leg ; more obvious purposes sub- 

 served are strengthening and defending of the joint, and affording part of the attachment 

 of origin of gastrocnemial and peroneal muscles. 



Tai-Hiis. — (Fig. 27.) The seven bones of the tarsus proper conform very closely, in 

 individual characters and general disposition, to those of the human foot. The locking 

 of the head of the second metatarsal between the two lateral 

 cuneiforms obtains. Among the more prominent differences are, 

 the shape of the articular surface of the astragalus, permitting 

 freer motion at the ankle ; the shape of the ento-cuneiforni, 

 producing the divergence and open articulation of the hallux ; 

 and the presence of the ossicle, attached to the ento-cuneiform, 

 for the support of the rudimentary spur. 



The astrrujahis (fig. 27, a) is depressed, and of an irregularly 

 triangular shape. The superior articulating surfiice — convex in 

 every direction — receives the impress of both tibia and fibula. ^ 



Along- the middle is a slight antero-posterior groove, limited on F'g- 27.— Tarsus and heads of 



, , ,. , • T mi • • 1 1 1 p metotarsals, 1 1-2 n.itural size. 



either side by a slight ridge. This groove receives the head oi 



the tibia, the internal malleolus resting against an oblique articular surfiicc on the inner 

 side of the groove ; and the external malleolus resting upon a similar, though shorter and 

 more convex, articular surface on the opposite side of the groove. The planes of these 

 two lateral articular facets are but few degrees oblique to that of the median one ; instead 

 of being, as in many mammals, nearly perpendicular. As a whole, the large convex artic- 

 ular surf ice calls to mind that formed by the proximal row of carpal bones ; and allows the 

 same kind, and nearly the same degree, of motion that exists in the wrist. The posterior 

 border of the astragalus is nearly straight; the internal one much curved; the antero- 

 external one deeply excavated. The lengthened outer corner overlies and rests upon the 

 calcaneum, near the middle of the latter ; the anterior corner also rests in part upon the 

 same bone, aiid extends to the cuboids, but is mostly received in, and presses upon, the 

 navicular, transmitting pressure to the cuneiforms. The posterior corner projects freely 

 backward and dips downward, to complete, -with the os calcis, the groove for the passage of 

 tendons behind the heel to the sole. The under surface is very irregular ; there is a slight 

 cupping for reception of a calcaneal protuberance, a decided convexity that fits a corres- 



