J. W. HULKE ON ORNITHOPSIS. 753 
than now. The under surface of the centrum is flattened transversely 
(this flattening may have been increased by pressure, but it has 
not, I believe, been wholly caused by it), whilst longitudinally it is 
made very concave by the swelling of the centrum towards the 
articular ends. The sides of the centrum are excavated by a large 
chamber, which is its most striking feature. The external opening 
of this chamber is an oval not less than 3°5 inches long and 2-2 
high. The upper border of the opening is thinner and more arched 
than the lower border, which is stouter and nearly straight. Inside 
this opening the chamber extends forward into the convexity of the 
front end of the centrum, reaching to within a short distance of the 
articular surtace ; it stretches backwards almost to the surface of 
the posterior cup; it dips downwards to a lower level than the 
lower border of the opening ; and it rises above to the under surface 
of the floor of the neural canal which assists to form its roof. The 
depth of this large chamber, measured vertically from the chord 
of the lateral surface of the centrum, is about 1:4 inch; and the 
chambers in the opposite halves of the centrum approach one another 
so closely that they are separated only by a very thin partition-wall 
in the longitudinal median plane, which, below, rests on the lower 
part of the centrum, and above bears up the floor of the neural 
canal. On each side, near its middle, this partition-wall is strength- 
ened by a vertical buttress-plate, which also furnishes additional 
support to the floor of the neural canal and the roots of the neural 
arch. The annexed sectional diagrams will assist in forming a clear 
idea of this singular structure (figs. 2, 3). 
Fig. 2.—Transverse Section of Fig. 8.—Horizontal longitudinal 
the Trunk-centrum (fig. 1). Section of the Trunk-centrum (fig. 1). 
SS SSAMAH> 
a. Upper lip of side opening. 
d. Lower lip of ditto. 
c, ec. Chambers. 
p. Partition. J. Front. 
6. Buttress. ¢e, ec. Chambers. 
p. Partition. 
6. Buttress. 
The bony tissue forming the outer surface of the centrum, as also 
that which lines the chambers and composes the partition-wall and 
buttress-plate, has a fine grain; it is less smooth and polished than 
