30 



the smooth surface of the pterygoid plates, where it gradually suhsides : this ridge 

 forms the angular termination of the bony palate (ib. ») 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,^. They are exposed by fracture of the walls in the pro- 

 tuberances marked .r, x, in Plate XV. 



From the state of the sutures already alluded to, the limits of the superoccipital, 

 parietal, and frontal bones cannot be defined. Above the arched superoccipital ridge 

 (Plate XIII. fig. 2, >) there are two semielliptical rough depressions {a, a) for mus- 

 cular attachments; and, in advance of these, the upper surface of the cranium shows 

 two other similar but shallower muscular impressions, b, b. The smooth surface of the 

 parietal, gradually narrowed to an inch in width ( 7) between the temporal ridges (t), 

 again as gradually expands into the frontal region (n), and it is perforated, a little 

 anterior to the middle of the temporal fossa, by a submedian vascular (venous?) 

 foramen (v), about half an inch in diameter. The temporal fossae (Plates XII. & 

 XIII. v, t, is) are remarkable for their great antero-posterior extent, and for the 

 encroachment upon them by the peculiar process (c) sent upward and backward 

 from the malar bone, as. They are partially defined anteriorly by the extension of a 

 postorbital process (Plate XII. fig. 1, is, Plate XIII. fig. 2, 12) downward to the malar 

 bone(j«); but, beneath and within this slender process, they communicate freely with 

 the orbits, 0. The posterior boundary ridge, continued from that on the parietal bone, 

 curves forward below and is continued into the sharp upper border of the zygoma, 

 Plate XII. fig. 1,17. The surface of the temporal fossae is grooved and perforated 

 posteriorly by large vessels, and is everywhere strongly impressed by the attachments 

 of muscular fasciculi. The base of the zygomatic process of the temporal bone has 

 an extensive origin (Plate XIII. fig. 2, v), not less than 6 inches in antero-poste- 

 rior extent ;* its free portion, to where it joins the malar, being 3 inches in length. 

 It is a strong trihedral bar of bone, rather concave on the upper and outer sides, and 

 forming on the underside the glenoid articular cavity for the lower jaw. This cavity 

 (Plate XV. g), of an oval form with the long axis transverse, measures 3 inches by 

 2§ inches, and is half an inch in depth. Behind this cavity the base of the zygoma 

 (Plate XII. fig. I,*?) has coalesced with the mastoid (s) and petrosal (i«) elements of 

 the temporal, which combine to form the meatus auditorius externus, e. This canal 

 is subcircular, about 10 lines in diameter at the deeper part, where it is formed by 

 the above elements, but doubtless wider at its outer part, where it was completed by 

 the tympanic bone. This bone is wanting in the skulls of the Megatherium hitherto 

 transmitted to England : the absence of any fractured surface upon the contour of 

 the orifice of the auditory canal indicates, however, that the bone was a free element 

 of the temporal in the Megatherium as in the Mylodon* and Glossotherium-f*. 



* Description of the Skeleton of an Extinct Gigantic Sloth (Mylodon robustus, Owkn), 4to, 1842, p. 28. 

 t Zoology of the Voyage of the Beagle, * Fossil Mammalia,' 4to, 1840, p. 59. pi. 16. fig. 4. 



