EDENTATA OF THE SANTA CRUZ BEDS. 23 



cess ; the narrowest portion is just behind the cotyles of the axis and 

 thence the mass widens posteriorly, though the widening is due princi- 

 pally to the coalesced transverse processes, the centra remaining of nearly 

 uniform breadth and very much depressed ; the mass has a faintly marked 

 median, ventral keel, which is best developed on the axis, and on each 

 side of this keel a number of obscure, oblique lines, converging forward ; 

 the centrum of the fourth cervical is made opisthoccelous by the projection 

 of its dorsal and ventral edges, though transversely it is plane. The 

 odontoid process of the axis is slender and of depressed cylindrical, rather 

 than conical form, and on its ventral side is a well-defined, saddle-shaped 

 facet for the inferior arch of the atlas. The cotyles for the latter are low, 

 broad and convex in both directions. In one of the specimens, but not 

 in the other, the dorsal border is notched for the vertebrarterial canal, 

 which opens just behind it and has a slightly more elevated position. 

 The neural canal is quite high and of trihedral shape at the front end of 

 the axis, becoming very low and broad on the fourth cervical, so that the 

 combined neural arches slope downward and backward. The neural spine 

 of the axis and the third cervical are fused together into one large, hatchet- 

 shaped plate, much the greater part of which is contributed by the axis. 

 In shape this compound spine seems to differ somewhat in the two speci- 

 mens, though in neither is it sufficiently complete to admit of exact com- 

 parison. On the fourth cervical the spine is very low, almost obsolete, 

 and is not united with that of the third vertebra. The coalesced zyga- 

 pophyses make a continuous ridge on each side of the neural arch, en- 

 larged at the points of union between the vertebrae and becoming espe- 

 cially conspicuous on the fourth. The transverse processes also form 

 continuous ridges, ending behind in large, simple, diverging and rod-like 

 projections, but for most of their length they are depressed, shelf-like 

 plates, with thin edges. Thus, the vertebrarterial canal is a long tube, 

 the large and conspicuous anterior opening of which is just behind the 

 cotyle of the axis, while the posterior opening is concealed in the trans- 

 verse process of the fourth cervical. On the ventral side of the trans- 

 verse process each vertebra of the compound mass has a foramen opening 

 into the vertebrarterial canal ; these foramina seem to be peculiar to the 

 present genus. The spinal nerves make their exits through foramina, 

 which in the axis perforate the pedicles of the neural arch, but elsewhere 

 have an intervertebral position. The fifth cervical is shorter and broader 



