14 MEGALONYX. 



size ; the fourth socket is too much mutilated to allow of a correct opinion being 

 formed as to the shape of the tooth which it contained." 



When the skull of the Megalonyx Jeffersonii in the collection of Dr. Dickeson, was 

 first submitted to my inspection, from the large size of the first molar tooth, and its 

 unusually advanced position from the other molars, I inferred the lower jaw above 

 indicated could not belong to the genus Megalonyx. Further, the recedence poste- 

 riorly of the two sides of the lower jaw is much greater than it possibly could be in 

 Megalonyx Jeffersonii, for according to Professor Owen's figure (PL xxix), repre- 

 senting the specimen reduced to two-thirds the size of nature, the distance between 

 the alveoli of the two sides at any position is over four inches, whereas in the upper 

 jaw of the skull of Dr. Dickeson's collection, the corresponding interval does not 

 measure more than an inch and three-quarters. 



Under these circumstances, as the lower jaw also differs in well-marked charac- 

 ters from that of Mylodon, Scelidothermm, &c., at an early period in preparing this 

 memoir, I proposed for the new genus indicated by the specimen, the name Qna- 

 thopsis, and dedicated the species to Professor Owen under the name of O. Oweni} 



These views are entirely confirmed by the specimen of a lower jaw accompanying 

 the Megalonyx skull in the collection of Dr. Owen. 



In its general form the inferior maxilla of Megalonyx is like that of Mylodon and 

 the recent sloths. 



The outer side of the body of the jaw (PL I), is vertically and antero-posteriorly 

 convex. Its front is relatively narrow compared with that of Mylodon and the Ai, 

 and at the lower part is convex ; but above it forms a prominent, pyramidal, keel- 

 like ridge to the symphysis. 



Over the position externally of the first alveolus the surface is uniformly convex, 

 but at its upper part appears more so from the existence of a deep concavity between 

 the first and second alveolus, and the presence of the anterior symphysial ridge. 



The inner surface of the body of the jaw (PL V, Fig. 2), is a vertical plane; and 

 the base is thick and convex. 



The alveolus for the first molar is three and a half inches in depth, and is directed 

 upward, forward, and outward; and its orifice presents a corresponding obliquity to 

 that above. 



The alveoli for the posterior three molai"s occupy a tract about three inches in 

 length, by fourteen lines in width, and they descend to the base of the jaw. (PL 

 V, Fig. 1). The hiatus separating them from the first molar is twenty lines long, 

 and is constituted by a thick perforated border curving a little outward in its course 

 forward. 



The ramus is a broad, thin plate, which appears as if it was inserted into the 

 body of the jaw on a line with the third molar alveolus. Its outer face is a vertical 

 plane, and presents a well marked ridge for muscular attachment, sweeping from 

 the anterior margin of the coronoid process in a semi-circle to the bottom of the 

 angular process. 



» Proc. Acad. Nat. Sci., VI, lit. 



