FOSSIL MAMMALIA. 71 



divided by the transverse constriction, the anterior is the largest ; their propor- 

 tions and oblique position are pretty accurately given in the figure. The longi- 

 tudinal diameter of the transverse section of this tooth is one inch, seven lines, 

 its greatest lateral or transverse diameter is ten lines, its least diameter at 

 the constricted part is three lines, the length of the entire tooth is four inches. 

 Judging from the form of the jaw, the length of the other teeth decreases in 

 a regular ratio to the anterior one. The posterior tooth is slightly curved, as 

 shown in fig. 2, PL XIX., with the concavity directed towards the outer side of 



the jaw. 



The general form of the horizontal ramus of the jaw, is so well illustrated in 

 the figures PL XVIII. and XIX., that the description may be brief. 



The symphysis is completely anchylosed, about four inches in length, and 

 extended forward to the extremity of the jaw at a very slight angle with the 

 inferior border of the ramus : it is of great breadth, smooth and gently concave 

 internally, and suggests the idea of its adaptation for the support and gliding move- 

 ments forwards and backwards of the free extremity of a long and well-developed 

 tongue. 



The exterior surface of the symphysis is characterized by the presence of two 

 oval mammilloid processes, situated on each side of the middle-line, and about 

 half way between the anterior and posterior extremes of the symphysis. A front 

 view of these processes, of the natural size, is given in fig. 4, PL XIX. : a side 

 view of the one on the right side represented in the reduced figure. 



Nearly four inches behind the anterior extremity of the above process is the 

 large anterior opening of the dental canal : it is five lines in diameter, situated 

 about one-third of the depth of the ramus of the jaw from the upper margin. 

 The magnitude of this foramen, which gives passage to the nerve and artery of 

 the lower lip, indicates that this part was of large size ; and the two symphyseal 

 processes, which probably were subservient to the attachment of large retractor 

 muscles, denote the free and extensive motions of such a lip, as we have presumed 

 to have existed from the size of the foramina destined for the transmission of its 

 nervous and nutrient organs. 



The angle of the jaw is produced backwards, and ends in an obtuse point, 

 slightly bent upwards; a foramen, one-third less than the anterior one, leads from 

 near the commencement of the dental canal, to the outer surface of the jaw, a 

 little below and behind the last molar tooth ; this foramen presents the same size 

 and relative position on both sides of the jaw. I find no indication of a corre- 

 sponding foramen, or of symphyseal processes in the figures or descriptions of 

 the lower jaw of the Megatherium, nor in the lower jaw of the Sloths, Ant-eaters, 

 Armadillos, or Manises, which I have had the opportunity of examining with a 

 view to this comparison. 



