MEGALONYX. 45 



sity at the bottom of the claw-procef?s, but in the other specimen upon one side it 

 remains entire. In this the Lateral portion of the ungual sheath is in the form of 

 an oval disk, with thin edges, attached below to the basal tuberosity. Its outer 

 surface is convex and perforated; and its posterior edge is unattached to the arti- 

 cular margin of the bone, indicating that the latter obtains its full development of 

 length before the ungual sheath incloses entirely the root of the nail. 



The two large vascular' foramina of the basal process in the mature phalanges, 

 in both young specimens are in the form of notches continuous with the separation 

 of the ungual sheath from the articular margin of the epiphysis. 



The nail which is preserved on the smaller of the specimens of the two 

 phalanges, is brown in color, is readily separable into its structural laminaB; and 

 it has the same form as the claw-process, except that its lower margin is grooved 

 as in the nails of the recent sloths. 



IVIegalonyx dissiiiiilis, Leidy. 



The collection of Dr. Dickeson contains two specimens of teeth found in the 

 ravines in the neighborhood of Natchez, Mississippi, which, though belonging to 

 tlie genus Megalonyx, apparently do not belong to the same species as that just 

 described. One of the teeth is a first molar, which I suspect to belong to the 

 lower jaw, as the wearing of the triturating surface is different from that of the 

 corresponding upper tooth of Megalonyx Jeffersonii. The other is probably a fifth 

 molar of the upper jaw, and in the description will be so designated. 



The specimen of the first molar (PI. XIV, Figs. 4, 5) is two and a half inches 

 long, but its lower part is broken away, and as the margin of the pulp cavity ante- 

 riorly measures two lines in thickness, when the tooth was perfect it probably was 

 an inch or more longer. The bottom of the pulp cavity is twenty lines from the 

 triturating surface, which, if we may judge from the fii'st molars contained in the 

 two skulls described of M. Jeffersonii, indicates the animal, at least, not to have 

 been an old one. In transverse section (PI. XVI, Fig. 8), the tooth is elongated 

 elliptical, with the outer side convex, and the inner side concave with a median 

 convex bulge. Its long diameter is seventeen lines, its short diameter seven and a 

 half lines. The tooth is relatively less curved longitudinally than in the corre- 

 sponding upper tooth of M. Jeffersonii. The triturating surface (PI. XIV, Fig. 5) 

 is worn into a deep concavity open to the bottom internally. Externally also the 

 cementum is worn off a short distance from the harder dentine. 



The structure of the tooth is the same as in the corresponding upper tooth of M. 

 Jeffersonii; but the cementum and harder dentinal layer are thickest externally, 

 and both together gradually become thinner to the middle line internally. 



In Cuvier's plate of bones of the Megalonyx, in the Annales du Museum,^ and in 

 the successive editions of the Ossemens Fossiles," is the representation of a trans- 



' V, PI. xxiii. » Ed. 3, PI. xv. 



