MEGALONYX. 19 



tively much less thick internally than those above described, and is also impressed 

 with a longitudinal groove internally. Its triturating surface is worn into a trans- 

 verse angular groove, the anterior and posterior margins of which are acute and 

 have an oblique course reverse to each other. Probably this tooth may belong to 

 a different animal from the Megalonyx. Its transverse diameter is twelve lines ; 

 and its antero-posterior diameter externally six lines, internally seven and a half 

 lines. 



The specimen of a small molar, accompanying the first superior molar and appjv 

 rently belonging to the same individual, from Natchez Bluffs, Mississippi, in the 

 collection of Dr. Wyman, appears to be a superior third molar ; but its section (PI. 

 XVI, Fig. 12), approaches more the quadrate form than that of any of the corre- 

 sponding teeth which have been described. Its triturating surface presents a deeper 

 concavity than in any of the foregoing specimens of posterior superior molars. The 

 transverse diameter of this tooth is eleven lines ; its antero-posterior diameter ex- 

 ternally six lines, internally seven and a half lines. 



The isolated upper second molar (PI. XVI, Fig. 14), from Natchez Bluffs, Mis- 

 sissippi, in the collection of Dr. Wyman, has nearly the form of the corresponding 

 teeth in the skulls above described. It is broader transversely, and narrower 

 antero-posteriorly, is less convex internally, and is impressed on its other three 

 sides. Its transverse diameter is ten lines ; its antero-posterior diameter externally 

 six lines, internally seven and a half lines. 



Of the two molars preserved in the lower jaw (PI. V), of Dr. Owen's collection, the 

 second of the series in transverse section (PI. XVI, Fig. 16), is quadrate with rounded 

 angles. The anterior and posterior sides are slightly convex, and the latter is the 

 broadest. The lateral sides are smaller than the others, and are about equal, and 

 they are slightly impressed at their middle. The triturating surface is concave, 

 and bounded by an acute edge, which is most prominent antero-internally and at 

 the postero-external angle. 



A section of the socket for the third inferior molar is also quadrate, with rounded 

 angles ; and the anterior and posterior sides are broadest and about equal. 



The fourth molar (PI. VI, Figs. 10, 11 ; XVI, Fig. 17), like the preceding pair, 

 is quadrate with convex sides ; the outer one being oblique and slightly impressed. 

 The triturating surface of this tooth (PI. VI, Fig. 11), is worn into a transverse 

 valley, whose boundaries are most prominent at the antero-internal and postero- 

 external angles. 



Lines. 

 Transverse diameter of the second lower molar . . . . . . .11 



Antero-posterior diameter of the second lower molar ...... Si- 

 Transverse diameter of the fourth lower molar . . . . . . -Hi 



Antero-posterior diameter of the fourth lower molar 8^ 



Structure of the Teeth. — As previously stated, the teeth of Megalonyx, as in all 

 sloth-like animals, consist of simple, long, fangless columns, deeply excavated fi'om 

 the bottom for the reception of a persistent dental pulp. 



At the commencement of the pulp cavity the teeth form an exceedingly thin 

 edge, and they very gradually increase in thickness to the apex of the cavity. 



