574 
PROFESSOR OWEN ON THE MEGATHERIUM. 
other part of the proper basis cranii, Plate XXL fig. 1, le: the petrosal, anterior to 
this, sends down a shorter rough pyramidal process. The carotid foramen (Plate 
XXIV. c), a full ellipse with diameters of 5 lines and 4 lines, is situated between the 
petrosal and basisphenoid at the fore-part of that oblong depression which is termi- 
nated behind by the large precondyloid foramen. 
The stylohyal (ss) has the form of a hammer, with a long, slightly bent handle, ter- 
minated by an obliquely truncated rough surface for syndesmosis with the ceratohyaL 
At the opposite end the handle is subcompressed, and the head is formed by a sudden 
expansion in the vertical direction, terminated posteriorly by a straight but rugged 
margin, and with the upper end produced, thickened, and forming a smooth con- 
vexity, or condyle, adapted to the cavity above described in the petromastoid. The 
lower end of the head or expanded part of the hammer-shaped bone is more pro- 
duced, more rugged, and terminates obtusely. The outer surface has a wide depres- 
sion at the middle, which is rough, with several short and well-marked ridges. The 
length of the specimen described is 8 inches, the breadth or depth of the expanded 
end is 3 inches and a half. 
The upper part of the coalesced frontals (Plate XXIL fig. 2, n) forms a smooth tri- 
angular plate, rapidly expanding to the postorbital processes (12) and very slightly 
convex. Some indistinct traces of the fronto-nasal suture seem to show that the 
nasal bones (Plate XXII. fig, 2, 15) extended backward beyond the transverse parallel 
of the postorbital processes ; more distinct traces of the naso-maxillary sutures (21), 
show that the coalesced nasals were 2 inches 9 lines across at their narrow posterior 
part, where they are flat above : at first slightly contracting, they then gradually ex- 
pand, and become more and more convex transversely to their anterior extremity. 
Here the nasal bones are also thickened, are rugged for the firmer attachment of the 
cartilaginous parts of the nose, and their under surface, being excavated by two longi- 
tudinal grooves, the thickened terminal surface is divided into a middle (Plate XXIII. 
fig. 2, m) and two lateral {n, n) parts, the latter being convex and subangular, and 
the middle expansion slightly excavated. As in the Two-toed Sloth {Choloepus didac-^ 
tyliis), the under surface of each nasal bone sends off a terminal plate or process for 
the attachment of a turbinal cartilage or ossicle. A narrow median groove indi- 
cates ihe original suture between the nasal bones along their anterior half. 
The cranial cavity of the Megatherium is considerably smaller than the cranial 
part of the skull, the outer wall or plate of bone being separated by large irregular 
air-cells from the vitreous plate, or that case of bone which immediately invested the 
brain and its membranes. The vertical diameter of the cranial cavity is 4 inches 
8 lines; its transverse diameter, which is greatest at the posterior third part of the 
cavity, corresponding with the posterior part of the cerebrum, is 6 inches. The 
brain of the Megatherium, to judge from its bony case, must have been less, by 
nearly one half, than that of the Elephant ; but with the cerebellum relatively larger 
and situated more posteriorly to the cerebral hemispheres : whence it may be inferred 
