442 Mr. Clift on the Megatherium. 



sessed by these substances, the grinding- surface of the tooth presents, in con- 

 sequence of attrition, four facets sloping from the two ridges of enamel which 

 traverse the tooth transversely ; the middle surfaces being the worn-down 

 ivory, the outer the worn-down caementum. 



In the anterior tooth the first facet is very small, from there being but little 

 caementum ; the second is almost perpendicular ; the third proportionally 

 more sloping than the others ; the fourth also nearly perpendicular. 



In the second and third teeth, the two middle surfaces are nearly equal, and 

 are worn down at the same angle. This is also the case with the two mar- 

 ginal surfaces, which, however, are much smaller tlian the middle ones. 



The secreting pulp, when the formation of the tooth had advanced to a 

 certain extent, appears to have assumed a most regular pyramidal or wedge 

 shape, and to have extended into one half of the length of the tooth. There 

 is no trace of a contraction at the open end of the cavity or root, but it is (as 

 in the incisors of Rodentia) wide open : this structure, therefore, indicates 

 perpetual growth, and evidently not a renewal by succession, as in the 

 elephant and mastodon, whose molar teeth are constantly advancing to the 

 front of both jaws an^ wearing out ; so that, though the component parts of 

 the teeth of the megatherium resemble those of the grinders of the elephant, 

 the mode of renewal is different, being like to that of the tusks of the same 

 animal. 



Vertebral Column. 



The very imperfect condition of the vertebral column is naturally a subject 

 of great regret : the atlas, however, is fortunateJy entire ; and as this affords 

 some data respecting the occiput, the diameter of the spinal chord, and, by 

 the size of the orifices for the vertebral arteries, also an indication of the 

 extent of the brain, it merits a more particular consideration. 



It is, as usual, in the form of a ring, being deficient in a body at the ante- 

 rior part, and having only the rudiment of a spine on the opposite aspect. 

 The articular surfaces for the condyles of the occiput are of an oval form and 

 concave, measuring in their longest diameter three inches three lines. The 

 opposite articular surfaces for the dentata are plane, and nearly circular; 

 and their diameter is two inches six lines. On the anterior part of the spinal 

 canal there is a heart-shaped surface for the rotation of the odontoid process ; 

 and on each side is a tuberosity for the attachment of the transverse ligament 

 which confines that process in its situation at the fore part of the spinal canal. 

 Each vertebral artery passed through a tortuous canal at the anterior part of 

 the vertebra, the diameter of which is ten lines. The spinal nerves also passed 



