41 



organization. In both these gigantic phyllophagous quadrupeds provision has like- 

 wise been made for the maintenance of the grinding machinery in an effective state ; 

 but the fertility of the Creative resources is well displayed by the different modes in 

 which this provision has been effected : in the Elephant, it is by the formation of 

 new teeth to supply the place of the old when worn out; in the Megatherium, by the 

 constant repair of the teeth in use, to the base of which new matter is added in pro- 

 portion as the old is worn away from the crown. Thus the extinct Megatherium had 

 both the same structure and mode of growth and renovation of the molar teeth, 

 as are manifested in the present day by the diminutive Sloths. 



§ 6. Comparison of the Skull and Dentition. 



The important affinity indicated by the dentition is confirmed by the characters 

 of the skull. In no other edentate family, save the Bradypodidae, is the cheek- 

 bone so nearly developed to the megatherioid proportions of that bone ; in no other 

 does it ascend above the zygoma into the temporal fossa or descend below the level 

 of the molar teeth. The large and complex malar bone is also associated, in the 

 Sloths, with a terminal position of the great anterior and posterior orifices of the 

 cranium, with terminal occipital condyles, and in the Ai {Bradypus tridactylus) with 

 a sloping occipital region. The cranial division of the skull is relatively as great in 

 the Sloths as in the Megatherium ; and the actual capacity of the cerebral cavity is 

 masked by a similar expansion of the air-cells, which almost everywhere surround 

 that cavity, and raise the outer plate of its bony parietes above the inner one. The 

 occiput presents the same expanded proportions, the same broad depressed basilar 

 plate, and the anterior condyloid foramina are of large relative size. The tympanic 

 is small ; it nearly completes a circular frame for the ear-drum, to which function it 

 is limited, and it long remains a separate bone. The detached and, in the skull de- 

 scribed, lost tympanies of the Megatherium have been evidently restricted to the 

 same office. The temporal fossa, in the Sloths, is long and large, and communicates 

 freely with the orbit, the outer boundary of which, however, is not completed in any 

 living species of Sloth. The nasals become confluent in old Sloths, and develope 

 turbinal laminae from their under surface. The premaxillaries are edentulous and 

 without any ascending process. The rami of the lower jaw expand and branch out 

 behind into a coronoid, a condyloid, and a long and deep angular process, and they 

 are anchylosed anteriorly at a broad sloping symphysis. Only in the genus Bradypus, 

 amongst known existing quadrupeds, do the alveoli of both jaws correspond in num- 

 ber, simplicity, relative depth and position with those of the Megatherium. The 

 still more important agreement between these existing and extinct Bruta, in the 

 peculiar structure of the teeth, yields the crowning proof that it is to the diminutive 

 arboreal Sloths that the gigantic Megatherium and its less bulky though larger extinct 

 congeners have the closest natural affinity. 



The chief differences observable in the cranial anatomy of the Sloths, as compared 



