29 



Each condyle (Plate XIV. fig. 1,», ») is a convexity of a subtriangular form, with 

 the base straight and the sides curving to an obtuse apex : the extent of their convex 

 curvature in the vertical or antero-posterior direction (Plate XII. fig. 1, 2) equals that 

 of a semicircle, and indicates that the Megatherium possessed considerable freedom 

 and extent of motion of the head. The parallel bases of the condyles are turned 

 toward each other and flank the sides of the foramen magnum, Plate XIV. fig. l,o. 

 This foramen is a wide or full ellipse with the longer axis transverse, and the plane, 

 through the greater prominence of the upper border, inclined forward and down- 

 ward at an angle of 135° with the basioccipital. The plane of the occiput forms, 

 with that of the basis cranii, an angle of 105°. The occipital region (ib. 1, 3, a, s) 

 is semicircular, its contour being formed by a thick rugged ridge continued from 

 the superoccipital (3) on each side to the mastoid, »: 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 spaces so 

 defined are roughened by muscular impressions. 



The paroccipital (Plate XIV. fig. 1, 4) is a moderately developed triangular tube- 

 rosity abutting against the back part of the articular cavity for the stylohyal (Plate 

 XV. ie), and separated by the outer occipital depression from the mastoid (s). The 

 precondyloid canals (ib. p), which transmit the motory-lingual or ninth pair of nerves, 

 begin by a large oblique aperture at the middle of the inn'er side of the base of the 

 condyles and extend along a course of 1\ inches in extent forward and outward to 

 open upon the back part of the rough fossa between the paroccipital (4) and petrosal 

 (i«) externally, and the basioccipital and basisphenoid internally, which fossa answers 

 to the • foramen lacerum in basi cranii ' of ordinary mammals. The carotid fora- 

 mina (c) open upon the fore-part of this fossa. The external 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 development of the tongue, and its great 

 use in stripping off the leaves and smaller branches of the trees affording the nourish- 

 ment of the Megatherium. 



The basisphenoid (Plate XV. 5) presents on each of its hinder angles a low 

 rugged subcircular protuberance (x) 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 XIV. fig. 1, n) is a long and nar- 

 row vertical oval, bounded above by a low transverse ridge (Plate XV. r), which 

 extends from the hinder border of the bony palate about two inches backwards upon 



B 



