scott: toxodonta OF THE SANTA CRUZ beds. 145 
The neck is short and heavy, considerably shorter than the skull. The 
atlas (PI. XXIV, figs. 5, 6) is very short antero-posteriorly, but extremely 
wide transversely, owing to the great development of the transverse 
processes, and quite depressed dorso-ventrally, and even aside from the 
processes, the width is the greatest of the three diameters. The anterior 
cotyles are very broad, deeply concave, quite widely separated ventrally 
and even more so dorsally. The neural arch is stout and shows only a 
moderate transverse convexity and, on each side, it bears a large and deep 
depression, the inner end of which opens into the foramen for the first 
spinal nerve and extends to the atlanteo-diapophysial foramen for the 
ventral branch of the same nerve. The neural spine is quite prominent, 
thick and rugose. The inferior arch is very like the neural, but smoother 
and less convex, and on the hinder border has a hypapophysis which 
varies much in size, though the difference may be specific rather than indi- 
vidual ; in some specimens it is very prominent and heavy, in others it is 
much reduced. The neural canal is low and wide, of transversely oval 
shape and has prominent rugosities for the attachment of the transverse 
ligament ; ventral to these is the large, semicylindrical concavity for the 
odontoid process of the axis. The posterior cotyles are low, very wide, 
of irregularly oval shape and with outer margins raised and projecting 
quite prominently. The transverse processes are very long, but rather 
narrow from before backward and posteriorly are quite thick, being 
strengthened by a broad bar, which runs out from the inferior arch over 
the processes. The large vertebrarterial canal perforates the transverse 
process and opens anteriorly into a large fossa, into which also opens the 
atlanteo-diapophysial foramen. 
In Toxodon the atlas is very much like that of Nesodon , but is relatively 
longer antero-posteriorly and less depressed, with more strongly convex 
neural arch. The neural spine is rather more prominent and is bifid, and 
the transverse processes are broader antero-posteriorly and have more 
curved free borders. The foramina are the same, except that the pos- 
terior opening of the vertebrarterial canal is much smaller and has been 
displaced toward the median line. The posterior cotyles have a greater 
dorso-ventral diameter and a more irregular surface, with depressions at the 
mesial ends, which are hardly indicated in Nesodon. 
The axis (Pis. XXIII, fig. 8 ; XXIV, fig. 4) has a short centrum, which 
anteriorly is very broad and depressed, but grows narrower and thicker 
