66 



ascribed to the singular extension and development of the air-cells, which 

 raise the outer considerably above the vitreous table, along the whole 

 upper plane of the skull. 



An explanation of the probable cause of these fractures, and a detailed 

 account and comparison of all the bones in the present unique skeleton, 

 are given in the separate work, published by the College, entitled 

 ' Description of the Skeleton of an extinct Gigantic Sloth (Mylodon ro- 

 bustus),' &c, 4to, 1842. Purchased. 



The following fossils to No. 470 inclusive belong to the species called My- 

 lodon robustus, and are from the same stratum and locality as the skeleton. 



378. The first molar tooth, right side, upper jaw. It is more curved, and is 



separated by a wider interval from the rest than they are from each 

 other, whereby it offers an obvious resemblance to the so-called canine 

 of the Two-toed Sloth (Cholcepus didactylus, Illig.), but it is not larger 

 than the rest of the molar series. The present example is much broken. 



Purchased. 



379. The first molar tooth, left side, upper jaw. A transverse section has been 

 taken from the crown or exposed part, and the surface polished. The 

 densest constituent shines most ; it consists of hard or unvascular dentine, 

 surrounds the trihedral axis of the vascular dentine, and is itself sur- 

 rounded by the thin coat of cement. The progress of carbonization has 

 followed the course of the calcigerous tubes of the dentine, which are 

 thus demonstrated to the naked eye. Purchased. 



380. The second molar, left side, upper jaw. The grinding surface has been 



broken off": the course of the calcigerous tubes of the hard dentine may 

 be discerned on the fractured surface with the aid of a pocket-lens. 



Purchased. 



381. A portion of the third molar, left side, upper jaw, including the grinding 



surface of the tooth. Purchased. 



382. A considerable portion of the fourth molar, left side, upper jaw; the 



grinding surface is nearly entire. Purchased. 



