572 
PROFESSOR OWEN ON THE MEGATHERIUM. 
foramen is a wide or full ellipse with the longer axis transverse, and the plane, 
through the greater prominence of the upper border, inclined backward and down- 
ward at an angle of 135° with the basioccipital. The occipital region (lb. 1,3, 8,3) 
is semicircular, its contour being formed by a thick rugged ridge continued from 
the superoccipital (3) on each side to the mastoid, s: the osseous wall included be- 
tween this arched ridge and the condyles ( 2 ), is divided into four shallow depres- 
sions, by a sharp median vertical ridge terminating below at a venous foramen just 
above the upper border of the foramen magnum, and by a pair of thick obtuse 
ridges (r) extending from the sides of the arched ridge obliquely inwards to near the 
apices of the occipital condyles, and thence to the paroccipitals (4). The depressions 
so defined are roughened by muscular implantations. 
The paroccipital (Plate XXIII. fig. 1 , 4 ) is a moderately developed triangular tube- 
rosity abutting against the back part of the articular cavity for the stylohyal (Plate 
XXIV. 16 ), and separated by the outer occipital depression from the mastoid (s). 
The precondyloid canals (Plate XXIV. ja), or those that transmit the rnotory-lingual 
or ninth pair of nerves, begin by a large oblique aperture at the middle of the inner 
side of the base of the condyles, and extend along a course of 2^ inches in extent 
forward and outward to open upon the back part of the rough fossa between the par- 
occipital (4) and petrosal (le) externally, and the basioccipital and basisphenoid in- 
ternally, which fossa answers to the ‘foramen lacerum in basi cranii’ of ordinary 
mammals. The carotid foramina (c) open upon the fore-part of this fossa. The ex- 
ternal precondyloid foramina {p, p) are situated on the inner side of the paroccipitals, 
are each an inch in diameter, and thus indicate the considerable muscular develop- 
ment of the tongue, and its great use in stripping off the leaves and smaller branches 
of the trees affording the nourishment of the Megatherium. 
The basisphenoid (Plate XXIV. 5) presents on each of its hinder angles a low 
rugged subcircular protuberance (<r) for muscular insertions ; in advance of which it 
slightly expands, the lateral margins inclining downwards so as to form the begin- 
ning of the smooth channel that contracts and deepens as it advances towards the 
posterior aperture of the bony nostrils. The broad subvertical pterygoid plates (24) are 
continued insensibly from the deflected margins of the basisphenoid ; they are smooth 
on the inside, irregularly channelled for muscular attachments on the outside. The 
posterior aperture of the bony nasal canal (Plate XXIII. fig. I, n) is a long and nar- 
row vertical oval, bounded below by a low transverse ridge (Plate XXIV. r), which 
extends from the hinder boder of the bony palate about two inches backwards upon 
the smooth surface of the pterygoid plates, where it gradually subsides : this ridge 
forms the angular termination of the bony palate {ih. 20) behind. The substance of 
the basisphenoid is excavated by air-sinuses, continued backward from the ordinary 
position of the sphenoidal sinuses, and extending into the basioccipital as far back 
as the jugular foramina, j. They are exposed by fracture of the walls in the pro- 
tuberances marked .r, x, in Plate XXIV. 
