49 



side of the bony arch, there are two very rough surfaces for the implantation of 

 the transverse hgament which bound down the odontoid process. The superior 

 margins of the posterior articular processes were separated, in the atlas figured, 

 from the upper arch by a deep groove, for the exit of the nerve. In another 

 mutilated atlas, transmitted with the bones described, but apparently not of the 

 same species of Mylodon, a small vertical bar of bone united the process with 

 the superior arch, and converted the groove into a canal. At the outer end of 

 this neural groove or canal, the posterior aperture of the vertebrarterial canal* 

 is situated : it soon divides into two smaller canals, one below and rather external 

 to the other. The upper canal perforates immediately the upper surface of the 

 transverse process, and appears at fig. 1. k; the lower canal first issues at the 

 lower surface at fig. 2. i, then perforates the upper surface, at fig. 1. h, where it 

 likewise seems to divide, into a canal which runs inwards to the spinal aperture, 

 and into a groove which is continued forwards and outwards. This complicated 

 condition of the arterial canal would indicate that the current of blood was in- 

 fluenced both by sudden bends of the artery, and by its bifurcation and probable 

 reunion at h, k. The broad upper or neural arch has a thick, rugged, obliquely 

 flattened anterior margin, and a thin, almost trenchant, posterior edge. 



The axis, or vertebra dentataf, has a long, subdepressed body, terminated 

 posteriorly by a vertical, elliptical and nearly flat articular surface, and pro- 

 longed anteriorly into the thick odontoid process, which is obliquely trun- 

 cated below to form the articular surface resting on the body of the atlas. The 

 neural arch springs from about half the extent of this long basis, near its pos- 

 terior end : it immediately sends off from its anterior and outer part a thick and 

 short round articular process, with a very slightly convex terminal surface, d ; 

 and behind this a short and slender transverse process, c, perforated at its base 

 by the vertebral canal, h. Above these the sides of the arch contract and con- 

 verge to the base of the large spinous process : this is prolonged on each side 

 into the posterior articular process, e, which is almost horizontal and looks down- 

 wards. The thick spinal plate expands antero-posteriorly as it ascends, over- 

 hanging the atlas by its rounded anterior angle, and covering the spine of the 

 third cervical vertebra with its more produced and sharper posterior angle. 



* Fig.S.g. t PI. VII. fig. 5. 



