MEGALONYX. 39 



wide at the middle, and expanded above and below. Tlie outer and posterior sur- 

 faces (PI. XII, Figs. 2, 3) are longitudinally concave, but transversely are convex. 



The head of the bone (PL XI, Fig. 4) presents the two femoral articular surfaces 

 separated by an uneven tract expanding anteriorly upon a broad, perforated convex 

 surface partially forming the upper part of the tuberosity for the ligament of the 

 patella. The inner articular surface is an oblique, irregularly oval concavity rising 

 at its anterior extremity into an angle of about 45°, and measuring in its long 

 diameter four inches, and in its short diameter three inches. The outer articular 

 surflice is a trilaterally oval, inclined plane, three and a quarter inches broad and 

 throe inches antero-posteriorly ; and it is continuous posteriorly -tvith a nearly 

 vertical convex, oval facet for conjunction with a sesamoid bone. 



The tuberosity for the insertion of the ligament of the patella is a prominent 

 convexity surmounting the acute outer margin of the bone, and forming the termi- 

 nation of a gradually expanding crest, which proceeds along the upper border of 

 the antero-internal face. 



The portion of the tibia which supports the articular surface for the outer 

 condyle of the femur, rises in the form of a strong abutment from its posterior 

 surface. It also supports the superior fibular articular surface, which is an ellip- 

 tical plane two and a half inches long and one and a third wide, directed down- 

 ward and outward. 



The distal end of the tibia is narrower than the proximal end. The articular 

 surface for the astragalus (PL XII, Fig. 4, h) is a transversely, oblong quadrilateral 

 concavity, measuring three and a half inches wide and two and three quarters 

 antero-posteriorly. At the middle antei'iorly it is bounded by a short pyr.amidal 

 process which fits into a corresponding fossa of the astragalus ; and externally it is 

 continuous at an obtuse angle with a trilateral plane for articulation with the 

 fibula. (Fig. 4, a.) 



The inner malleolus is a prominently convex tuberosity terminating the outer 

 border of the bone, and supporting posteriorly a conspicuous trochlear process, the 

 groove of which is directed downward and forward. 



Inches. 

 Length of the tibia at its antero-external surface . . . . . . .12 



Greatest length, which is from the contiguous margins of the femoral articular sur- 

 faces to the trochlear process of the malleolus . 15 



Greatest breadth of head ........... 8J 



Antero-posterior diameter at middle of the head 5^ 



Greatest breadth of tarsal end .......... 6|- 



Antero-posterior diameter ........... 3|^ 



Patella. — (PL XI, Figs. 5, 6.) The patella) of Megalonyx, in Dr. Owen's collec- 

 tion, present nearly the same form as in Mijlodon ; being trilateral with rounded 

 angles, the base upward and the apex downward. 



The base is thick, rough, and perforated. The apex is tongue-like in form, and 

 curved inwardly ; and it measures two inches and a half in length from the femo- 

 ral articular surface. The latter (Fig. 6) is transversely elliptical and convex, and 



