MEGATHERIUM. 53 



between the fifth or last pair of alveoli at their middle is thirty-two lines. The 

 under surface of the hard palate is concave transversely, especially towai'ds the 

 extremities ; is slightly convex antero-posteriorly, and is perforated with a great 

 number of conspicuous foramina. Between the position of the last molar alveoli, 

 large palatine canals commence at the termination of a larger channel proceeding 

 on each side from the posterior extremity of the palate. 



The antero-posterior measurements of the superior molar alveoli, so far as they 

 can be ascertained from the specimen just described, are as follows : — 



Lines. 

 Fii'st molar alveolus ............ 8 



Secoud molar alveolus . . . . . . . . . . . .16 



Third molar alveolus . . . . . . . . . . . .19 



Fourth molar alveolus 19 



Fifth molar alveolus .15 



The lower jaw of the Mrgatherinm, in the collection of the National Institute, ia 

 one of the most perfect and intei'esting specimens yet discovered in North America. 

 It, I believe, was presented to the Institute by Dr. J. P. Scriven. 



The base of the lower jaw (PI. XV, Fig. 1) of Megatherium, on a line with the 

 teeth, presents a remarkable convex extrusion to accommodate the extraordinary 

 length of the latter, which, at their bottom, are only two or three lines from the 

 exterior surface of the bone. The alveolar portion of the jaw is six inches in depth ; 

 and its outer side is convex and rough, and its inner surfiice nearly a vertical 

 plane and smooth. The ramus externally and internally is roughened with ridges 

 for the attachment of the powerful masseter and pterygoid muscles. 



The alveolar border of the jaw occupies a tract which is seven and a half inches 

 long, and two inches wide at the middle. The teeth are separated by intervals of 

 about the fourth of an inch, gradually widening outwardly and inwardly ; and they 

 are exserted in a gradually increasing extent from behind forward ; the last one 

 being about four lines above the alveolar border internally, and the first one a little 

 more than an inch. 



The dental canal commences at the root of the coronoid process about two inches 

 back of the last molar tooth ; one of its branches opening exterior to the latter, 

 and the other advancing towards the symphysis of the jaw. 



The teeth contained within the lower jaw just described are perfect; and they are 

 quadrilateral, curved columns with the angles rounded, the anterior and posterior 

 surfaces convex, and the lateral surfaces forming planes. The triturating surfaces 

 are worn into transverse, angular grooves, bounded before and behind with angular 

 ridges. The measurements of the teeth are as follows : — 



Inferior Molars. 

 First. Second. Xliird- Fourtli. 



Antero-posterior diameter 18 Hues. 19 lines. 20 lines. 18 lines. 



Transverse diameter . . . . . . 22 " 22 " -0 la 



Length in the eurve f inches, t inches. 7 inches. 6 inches. 



The annular metacarpal bone, of the National Institute collection, is nine and a 

 quarter inches long. Its shaft is quadrate, with the outer angles rounded, and the 



