w 



FOSSIL MAMMALIA. 



65 



comprehensive characters of kind and general structure of the teeth, or upon the 

 more restricted ones, of form and such modifications in the disposition and pro- 

 portions of the component textures of the tooth, as give rise to the characteristic 

 appearances of the triturating surface of the crown. 



With respect to existing Mammalia, most naturalists of the present, day 

 seem to be unanimous as to the convenience at least of founding a generic or sub- 

 generic distinction on well marked modifications in the form and structure of the 

 teeth, although they may correspond in number and kind, in proof of which it 

 needs only to peruse the pages of a Sy sterna Mammalium which relate to the 

 distribution of the Rodent Order. According to this mode of viewing the 

 logical abstractions under which species are grouped together, the extinct 

 Edentate Mammal discovered by Jefferson must be referred to a genus distinct 

 from Megatherium, and for which the term Megalonyx should be retained. 

 This will be sufficiently evident by comparing the descriptions given by Cuvier 

 of one of the teeth of the Megalonyx Jeffersonii, and by Dr. Harlan of a tooth of 

 his Megalonyx laqueatus, with those of the Megatherium which have been published 

 by Mr. Clift. The fragment of the molar tooth of the Megalonyx Jeffersonii, de- 

 scribed and figured in the Ossemens Fossiles, seems to have been implanted in the 

 jaw, like the teeth of the Megatherium, by a simple hollow base similar in form and 

 size to the protruded crown : its structure Cuvier describes as consisting of a 

 central cylinder of bone enveloped in a sheath of enamel.* The transverse section 

 of this tooth presents an irregular elliptical form, the external contour being 

 gently and uniformly convex, the internal one, undulating; convex in the middle, 

 and slightly concave on each side, arising from the tooth being traversed longitu- 

 dinally on its inner side by two wide and shallow depressions. 



The imperfect tooth of the species called by Dr. Harlan Megalonyx 

 laqueatus, and of which a cast was presented by that able and industrious 

 naturalist to the Museum of the. Royal College of Surgeons, resembles in general 

 form, and especially in the characteristic double longitudinal groove on the inner 

 side, the tooth of the Megalonyx Jeffersonii. It is thus described by Dr. Harlan : 



" The fractured molar tooth appears to have belonged to the inferior maxilla 

 on the right side ; the crown is destroyed ; a part of the cavity of the root remains. 

 The body is compressed transversely, and presents a double curvature, which ren- 

 ders its anterior and exterior aspects slightly convex ; the posterior and interior 

 gently concave ; these surfaces are all uniform, with the exception of the interior 

 or mesial aspect, which presents a longitudinal rib or ridge, one-half the thick- 

 ness of the long diameter of the tooth ; with a broad, not profound longitudinal 



* It is most probable that the substance which is here termed " enamel," is similar to that which forms the 

 dense prominent ridges in the tooth of the Megatherium, and which I have shown to be composed of minute 

 parallel calcigerous tubes, similar to the ivory or bone of the human tooth. 



K 



