21 



long, subdepressed, shuttle-shaped bone, perforated by a large aperture, quadrate 

 below for its detached centrum the ' odontoid process,' arched above for the spinal 

 cord. 



The thinnest and smallest part of the ring of the atlas is formed by the hypapo- 

 physis, hy, which has coalesced with part of the bases of the neural arch, nn, and 

 has supplanted, as it were, the proper centrum, o, Plate V. figs. 1 & 2, which has 

 remained anchylosed to that of the axis. The upper surface of the hypapophysis 

 presents a shallow articular surface, o, Plate IV. fig. 3, for that centrum to rest and 

 turn upon. The hinder half of the base of the neurapophysis developes, on each side, 

 a slightly concave, subcircular, articular surface, z', for a moveable articulation with 

 that on the side of the odontoid, zn, fig. 1, Plate V. The atlas is perforated an- 

 terior and external to this by a foramen, Plate IV. fig. 1, s, answering to that called 

 ' foramen alare posterius ' in the Horse, in which it gives passage to the posterior 

 branch of the occipital artery ; in the Megatherium the foramen or canal is bridged 

 over by a narrow oblique bar of bone, dividing its external outlet into two, r & s, 

 and through the hinder, s, of the divisions it is probable that a branch of the second 

 spinal nerve may have passed. 



The diapophysis, d, is a broad, depressed, rounded aliform process, with a protu- 

 berance from its under and back part, like the rudiment of a pleurapophysis, pi. 

 Anterior to this process the under surface of the diapophysis is deeply and widely 

 excavated and perforated by the vertebral artery, the canal for which, opening upon 

 the upper surface of the diapophysis, is then continued obliquely inward, perforating 

 at q, fig. 4, Plate IV. the upper part of the neural arch, just within the upper part of 

 the condyloid concavities. A large part of the canal for the first spinal nerve, fig. 1, v, 

 opens into the outer commencement of the vertebral canal, and answers to that 

 called 'foramen alare anterius' in the Horse, which transmits the inferior branch 

 of the first spinal nerve as well as the anterior branch of the occipital artery. The 

 condyloid concavities, fig. 2, nz, are semioval, large and deep, and occupy nearly the 

 whole of the anterior surface of the neural arch, being separated above by a rough 

 tract of three inches' extent, upon which the vertebral canals open. There is a tri- 

 angular rough surface at the back and inner part of each condyloid concavity. 



Such are the modifications of the different cervical vertebrae of the Megatherium. 

 With regard to the dorsal vertebrae, their chief characteristics may be briefly recapi- 

 tulated as follows : — 



The first dorsal vertebra is distinguished by the confluence of the neuro-costal and 

 dia-costal surfaces, and by the superior height of the spine. 



The second to the fifth dorsals inclusive, like the first, have only the ordinary pair 

 of zygapophyses before and behind, but have the neural and diapophysial surfaces 

 for the rib distinct. 



The sixth dorsal is recognizable by having a third median zygapophysis behind, 

 but not in front. The seventh to the twelfth dorsals inclusive have the three zyg- 



