44 MEGALONYX. 



of the same toe and of the second or fourth toe of both feet; and the first of the 

 last toe. 



In the third or median toe the homologues of the first and second phalanges form 

 a single bone (PL XIII, Figs. 17, 18), which is short and very robust. Its shaft is 

 quadrate, and decreases in diameter anteriorly. The proximal articular surface, as 

 in the first phalanges of the forefeet, presents a deep, vertical concavity, with an 

 oblique oifset on each side inferiorly. The distal extremity like that of the second 

 phalanges of the forefeet, forms a trochlea, the median groove of which is bounded 

 above and below by a fossa, to accommodate the movements of the last phalanx. 

 The sides of the trochlea are plane, but are deepened towards the centre, just pos- 

 terior to which is a tuberosity for the attachment of the lateral ligament. 



Inches. 

 Length of the homologue of the first and second phalanges of the middle toe, 

 laterally ............. S^ 



Depth of proximal extremity 3 J 



Depth of trochlear extremity . . . . . . . . . . 2 ' 



The last phalanx of the third toe (Figs. 19, 20), is like that of the corresponding 

 toe of the forefoot, but is very much larger and stronger. Its measurements are 

 as follows : — 



Inches. 

 Length in straight line ........... 8j^ 



Greatest depth 4^ 



The last phalanx of the second or fourth toe (Fig. 21), is like those of the cor- 

 responding toes of the forefeet. 



Inches. 



Length 6^ 



Depth ; . .3 



The first phalanx of the last toe is a quadrilateral nodule, with a concave metar 

 carpal facet, and a small, oblong, convex facQt for the next phalanx. 



Inches. 



Length . . f 



Breadth . . .1 



Depth 1| 



The two ungual phalanges of the hind foot of a young individual of Megalonyx 

 Jefferscmii, in the collection of the Academy of Natural Sciences, and forming part 

 of the specimens upon -which Dr. Harlan proposed the M. lagueatus, are interesting 

 as indicating the mode of their development. 



One of the specimens belongs to the median toe, and the other to either the 

 second or the fourth toe. 



In the median ungual phalanx, the line of separation of the articular epi- 

 physis is indicated by a fissure still existing at the lower third of the bone ; and 

 in the other ungual phalanx the articular epiphysis is yet separable. In the former 

 specimen, the ungual sheath is broken away except at its attachment to the tubero- 



