N 





102 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



to air, derived from the sphenoid sinuses, (PL XXX. fig. 2.) The vertical diame- 

 ter of the cranial cavity is four inches, eight lines; its transverse diameter, which 

 is greatest in the posterior third part of the cavity, corresponding with the pos- 

 terior part of the cerebrum is six inches : from the indications afforded by the 

 remains of the cranial cavity in Mr. Darwin's specimens, I conclude that the brain 

 of the Megatherium was more depressed, and upon the whole, smaller by nearly 

 one-half than that of the Elephant ; but with the cerebellum relatively larger, 

 and situated more posteriorly with relation to the cerebral hemispheres : whence 

 it may be concluded that the Megatherium was a creature of less intelligence, 

 and with the command of fewer resources, or a less varied instinct than the 

 Elephant. 



It has been usual to characterize the Megatherium, in conformity with the 

 concurrent descriptions of Bru, Cuvier, and D'Alton, by the dental formula of 

 molares | *, i. e. by the presence of four grinding teeth on each side of the upper, 

 as of the lower jaw. It was the agreement of the excellent authorities above 

 cited in this statement, which induced Mr. Clift and myself to regard a single 

 detached tooth, which formed part of the valuable collection of remains of the 

 Megatherium deposited in the Hunterian Museum by Sir Woodbine Parish, as 

 being, from its comparatively small size, the tooth of either a younger individual 

 or of a smaller species of Megatherium. Upon clearing away the matrix from the 

 palatal and alveolar surface of one of the cranial fragments of the Megatherium 

 in Mr. Darwin's collection, I was gratified by the detection of the crown of a fifth 

 molar, corresponding in size and form with the detached tooth, above alluded to : 

 its small size, and its position have doubtless occasioned its being over-looked in 

 the cranium of the great skeleton at Madrid. 



The anterior molar of the upper jaw presents a nearly semicircular trans- 

 verse section, with the angles rounded off; the three succeeding teeth are four- 

 sided, with the transverse somewhat exceeding the antero-posterior diameter : 

 they are rather longer and larger than the first : the last molar is likewise four- 

 sided, but presents a sudden diminution of diameter, and is relatively broader. 

 The following are the respective dimensions of the upper maxillary teeth. 



Length f 



Transverse diameter . 

 Antero-posterior diameter . 



First Molar. Second Molar. Third Molar. Fourth Molar. Fifth Molar. 



In. Lines. In. Liues. In. Lines. In. Lines. In. Lines. 



86 94 94 87 52 



24 23 2 14 



1 



9 

 5 



2 



2 



11 



10 



Besides the differences in size, the upper molars vary as to their curvature : this 

 difference is exhibited in the vertical section of these teeth figured in PL XXXI. 

 The convexity of the curve of the first, second and third molars is directed 



