584 
PROFESSOR OWEN ON THE MEGATHERIUM. 
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. 
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 Bradypodidce, 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 larainse 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 
ai’boreal 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 
with that of the Megatherium, are the greater relative depth and breadth, and the 
more convex outline of the coronal aspect of the skull ; but this difference would be, 
doubtless, much less marked in the immature than in the adult Megatherium. The 
zygomatic process, in the Sloths, is relatively shorter, and does not attain the malar 
bone ; this, therefore, has not the middle process for supporting the zygoma, and is two- 
pronged, instead of being, as in the Megatherium, four-pronged. The chief characters 
by which the Megatherium deviates, in its cranial structure, from the bradypodal and 
approaches to the rnyrmecophagal type, are the elongation of the slender edentulous 
