﻿I860.] 



HUXLEY MACEATJCHENIA BOLIVIENSIS. 



79 



and the height of the triangle from its apex, which corresponds with 

 the hase of the crest, to its base (the concave inner wall of the 

 cranium) is nearly 04 of an inch. 



In viewing the fragment of the occiput from within, one is sur- 

 prised by the great thickness of the supra-occipital region, the bone 

 immediately above the middle of the occipital foramen being half an 

 inch thick. A well-marked ridge, defining the interior boundary of 

 the cerebellar fossa, is continued downwards, forwards, and out- 

 wards, from the anterior boundary of the thick roof of the occipital 

 foramen. There is no venous canal traceable above the inner aper- 

 ture of the precondyloid foramen. 



If the occiput of Macrauchenia Boliviensis be restored by reversing 

 the outlines of the right half (as in PI. VI. fig. 3), thus supplying the 

 wanting left moiety, the following measurements may be obtained. 

 Side by side with them I give the corresponding measurements of the 

 skull of the Yicugna : — 



M. Boliviensis. Vicugna. 

 Transverse diameter of the occiput] in. in. 



from the outer edge of one para- I 1-9 2-25 



mastoid to that of the other .... J 

 Ditto from the outer edge of one occi-1 -^.5 



pital condyle to that of the other. J 

 The transverse diameter of the occi- 1 « 



pital foramen J 



It will be observed that the two series of dimensions correspond 

 very closely, the two latter being identical, while the Macrauchenia 

 appears to have had even a narrower skull than the Vicugna. In 

 form, the occiput of Macrauchenia agrees better with that of the 

 Llamas than with that of any other ungulate animal with which I 

 have compared it. 



Thus, in an old Guanaco I find an equally well-marked ridge in 

 the middle line of the supra-occipital element ; the occipital crest is 

 equally prominent, though not so stout ; the sagittal crest is as well 

 marked, thin and sharp, and, as in Macrauchenia, its superior edge 

 ascends. There is a fossa between the occipital condyle and the 

 paramastoid, similar in form to that in Macrauchenia, though much 

 shallower. The occipital condyles are very much alike ; and their 

 relation to the precondyloid foramina is the same in both cases. The 

 paramastoid has the same proportional breadth; and its greatest dia- 

 meter is, in both cases, directed from without and behind, inwards and 

 forwards : in both cases its inner edge is peculiarly thickened. Again, 

 the paramastoid of the Auchenia, like that of the fossil, is very short, 

 its apex hardly extending below the level of the occipital condyle. 



The occiput of Macrauchenia, on the other hand, differs from that 

 of Auchenia in the much greater thickness of the supra-occipital, 

 which in the Macrauchenia has fully double the thickness of the same 

 region in an old Guanaco, whose skull is much larger — in this respect 

 approaching the Sheep and some other Euminants, which have this 

 bone very thick. The supra-occipital, also, is much higher, in pro- 

 portion to its width, in Macrauchenia than in Auchenia ; its lateral 



