392 MR. ST. GEORGE MIVART ON THE 
and its two outer margins very slightly so; the transverse extent of the dorsal margin 
of this surface considerably exceeds that of its ventral margin. The ventral margin of 
the whole centrum is of course not only convex, but much produced ventrad by reason 
of the hypapophysial process. The extreme width of the articular surface of the 
postaxial side of the centrum exceeds its dorso-ventral extent only slightly. The 
ventral lip of the articular surface extends much further postaxially than does its dorsal 
margin (fig. 8, pc); but this extension is actually less, and relatively very far less, than 
the postaxial extension of the hypapophysial process of the atlas. 
The neural surface, even apart from the dorsal surface of the odontoid process, which 
continues it on preaxiad, is exceedingly prolonged compared with that of the atlas. 
Nearly straight antero-posteriorly, it is slightly concave transversely. 
Certain small irregular foramina may lead into the substance of the centrum from 
nearly the postaxial extremity of this neural surface, the postaxial margin of which is 
slightly convex. 
The ventral surface of the centrum is nowhere concave, but is greatly produced ven- 
trad medianly, except at and towards its preaxial end. Its preaxial margin is nearly 
straight, but may be very slightly convex or very slightly concave; its postaxial margin 
is narrower, but is very strongly convex (fig. 12). 
The median production before referred to isa strong hypapophysial (figs. 8, 12, hy) 
ridge, which, beginning a little preaxiad of the antero-posterior middle of the ventral 
surface of the centrum, rapidly increases in prominence postaxially till near the post- 
axial margin, when it yet more suddenly subsides. Its ventral margin is nowhere 
pointed, but presents a curve, the preaxial slope of which is more gradual than the 
postaxial one. 
As the hypapophysial ridge increases and decreases in dorso-ventral extent, it also, 
but more gradually, increases and decreases in its transverse dimension. Its ventral 
surface, however, is not flattened out. 
Two ridges (fig. 12, 7) traverse obliquely the preaxial part of the ventral surface of 
the centrum, diverging postaxially from a little behind its preaxial margin to the para- 
pophysial part hereafter to be noticed. These ridges seem to be the homotypes of those 
which run preaxiad from the lateral hypapophysial tubercles on the ventral surface 
of the atlas vertebra. 
The odontoid process is between three and four times the size of the hypapophysial 
process of the atlas vertebra. Its preaxial margin is rounded ; its upper surface is flat or 
slightly concave, continuing on preaxially the dorsal (or neural) surface of the centrum. 
Its ventral aspect presents three surfaces. The most postaxial of these three is the con- 
tinuation preaxiad of the preaxial articular surface of the centrum; and it is this part 
which articulates with the little articular surfaces prolonged preaxiad on to the post- 
axial part of the dorsal surface of the quasi-centrum of the atlas. 
The median surface of the three is irregularly excavated and probably gives attach- 
