﻿I860.] 



HUXLEY — MACEATTCHENIA BOLIYIENSIS. 



77 



that, as in other bones, the proportions of diverse diameters of the 

 same bone are not the same in the two species. But as the trans- 

 verse diameters of the cervical vertebra? of the two species are nearly 

 as 1 : 3, and the transverse diameters of the lumbar vertebrae are, 

 also, nearly in the ratio of 1 : 3, it would seem as if the different 

 regions of the vertebral column of the two species exhibited the same 

 proportional correspondence to one another. 



The skull. — As no part of the skull of Macrauchenia Patachonica 

 has yet beeu discovered (with perhaps the exception of part of the 

 lower jaw), a great interest attaches to every fragment which promises 

 to throw light upon this part of its organization ; and I therefore make 

 no apology for dwelling at some length upon the characters of the 

 two very imperfect and mutilated portions of the cranium which 

 turned up among the specimens submitted to me by Mr. Forbes. 



The one of these (Plate VI. fig. 3) consists of rather more than half 

 of the occipital segment of the skull, and exhibits the whole of the 

 supra-occipital bone, with its strong occipital crest, a part of the 

 parietal with the sagittal crest, the greater part of the right para- 

 mastoid process, and the entire right occipital condyle. 



As I have already remarked, the sutures are obliterated : and this is 

 true, not only of those which ordinarily exist between the elements of 

 the occipital bone in young mammals, but of the lambdoidal suture, 

 which usually persists for a longer period. The occipital foramen must, 

 when entire, have had a depressed-oval form, the short, vertical axis 

 of the oval being about - 6 of an inch long. The face of the bone 

 above it iuchnes upwards and forwards, at an angle of about 50° with 

 the base of the skull, and presents a sharp ridge in the middle line, 

 on either side of which the surface of the supra-occipital element slopes 

 with a slight convexity outwards and forwards, at the sides and below; 

 while, above, it becomes concave by passing almost vertically upwards 

 in the middle line, and laterally, bending upwards and backwards 

 at a right angle with its previous inclination into the occipital crest. 



This crest is nearly 0-2 inch thick at the sides, and becomes still 

 thicker in the middle line, where it joins the sagittal crest. It Is 

 1*1 inch in diameter at its widest part, and about half an inch high. 

 Its contour is that of a parallelogram, with its angles rounded off, 

 and the middle of its upper side rather truncated. The lateral 

 portions project backwards rather more than its centre ; so that, while, 

 supposing the basi- occipital to be horizontal, a vertical line drawn 

 through the posterior edge of that bone would nearly coincide with 

 the contour of its central part, it would pass a little anterior to the 

 plane of the lateral extremities of the crest. Inferiorly, the thick 

 lateral portions of the crest divide into two ridges ; the posterior 

 of which turns slightly inwards and comes to an end, while the 

 anterior, much sharper at its edge, passes forwards and outwards, and 

 becomes continuous with the sharp ridge in which the paramastoid 

 process terminates externally. 



Behind this ridge, between the paramastoid process, the occipital 

 condyle, and the lateral convexity of that part of the occipital bone 

 which lies above the foramen magnum, there is a deep fossa, which is 



