MEGALOXYX, 17 



collection. The measurements of a cast of this tooth are sixteen and a half lines 

 in its antero-posterior diameter, and nine lines in its transverse diameter. 



On commencing my investigations of Megalonyx, with only two si:)ecimens of 

 first upper molar teeth befoi'e me, in addition to the one contained in the skull of 

 Dr. Dickeson's collection, I found such differences of form as to lead me to the 

 conclusion that they belonged to three distinct species, while a fourth was based 

 upon the description of a tooth by Dr. Harlan.^ Further discoveries have indi- 

 cated a considerable extent of variation in the form of the same teeth in different 

 individuals, so that I now feel convinced that at most there are not more than two* 

 known species of North American Megalonyx, and perhaps only one. 



As before stated, the first molar is lost fi'om the lower jaw in Dr. Owen's collec- 

 tion ; nor have I had an opportunity of inspecting a first lower molar tootli of the 

 Megalonyx. A cast, which I made in clay of the first molar alveolus in the lower 

 jaw above mentioned, presents a form like that of the corresponding teeth of the 

 upper jaw, but it is a little more convex externally, and is of more uniform thickness, 

 or its inner median bulge is less prominent, and anterior and posterior to this it is 

 thicker. Its antero-posterior diameter is seventeen lines ; its greatest transverse 

 diameter is nine lines. (PI. XVI, Fig. 7. Section of the cast.) 



The tooth represented by Cuvier in figui'e 14 of the plate of the bones of Mega- 

 lonyx, in the Ossemens Fossiles, judging from the view of the triturating surface, 

 appears to me to be the first lower molar of the right side. In the corresponding 

 upper molars, the triturating surface is nearly concealed when the teeth are viewed 

 upon the inner side. 



The posterior four molars (PI. Ill), in the skull of Dr. Owen's collection, where 

 nearest together are separated by an interval of several lines, which widens more 

 or less towards the inner and outer sides of the teeth. The median pair are the 

 largest and longest. 



The second and third molars are nearly straight in their course, but incline very 

 slightly outward ; the fourth molar in addition, curves slightly backward ; and the 

 last of the series curves strongly backward in its course downward. 



The second upper molar in section (PI. XVT, Fig. 9, d), is quadrate with rounded 

 angles. Its inner and posterior sides are the greater, and are nearly equal ; and the 

 remaining sides are also nearly equal, and are planes. The inner side is slightly 

 convex, and the posterior side is nearly a plane and is directed obliquely outward. 



The third and fourth molars are relatively much broader compared with their 

 measurement antero-posteriorly, than in the second tooth of the series. In section 

 (PI. XVI, Fig. 9, c, b), they form an isosceles triangle with the base internal and 



> Megalonyx Jeffersonii, M. laqueatus, M. potens, and M. dissiviiUs. See Proc. Acad. Nat. Sci., 

 VI, 1852, 117. The first upper molar in the skull of Dr. Dickeson's collection, I viewed as charac- 

 teristic of Megalonyx Jeffersonii; that figured by Dr. Harlan (Journ. Acad. Nat. Sci., VI, PI. XII, 

 Figs. T-9; Med. and Ph3'S. Res., PI. XII, Figs. 7-9), as characteristic of M. laqueatus; that repre- 

 sented in PI. VI, Figs. 6, 7, and PI. XVI, Fig. 3, of this memoir, as characteristic of M. potens; and 

 that represented in PI. XIV, Figs. 4, 5, 6 ; PI. XVI, Figs. 8, 15, as characteristic of M. dissimilis. 



" Megalonyx Jeffersonii, and M. dissimilis. 

 3 



