ANATOMY OF HETERODONTUS: ENDOSKELETON 467 
nerve (f.v.), the latter to the dorsal root (j.d.) of the first spinal 
nerve. Capping the column between the first and second inter- 
dorsal plates on each side is a smaller plate—the suprabasidorsal 
(s.b.d.) or neural spine. The centrum itself has extending from 
its side a basiventral (b.v.) from which the rib (r.) projects. 
In an end view of the fifth vertebra the structures there con- 
cerned are seen to advantage (fig. 18). Resting upon the cen- 
trum is the neural arch (n.a.), the sides and the top of which 
are made up entirely of the cut edge of the interdorsals (7.d.), 
but the basidorsal plates (b.d.) of the vertebra appear in this end 
view. At the ventro-lateral angles of the centrum are the ribs » 
(r.) which stand out almost at right angles. 
A similar view taken of a vertebra toward the tail region, say 
the fifty-third, in addition to a neural arch above, shows a haemal 
arch below the centrum (h.a., fig. 21). The neural arch in this 
case shows the interdorsal plate with a basidorsal (b:d.) seen in 
end view. The haemal arch is formed by the downward bending 
of the basiventrals and by the adjoined haemal spine; the basi- 
ventrals make up the larger part of the wall and all of the floor of 
the canal. Contiguous to the centrum, however, are seen the 
interventral plates of the vertebra. In such a view it will be 
observed that the haemal canal is divided by a partition into a 
dorsal and a ventral part, the dorsal being for the caudal aorta 
and the ventral for the caudal vein. 
If a series of transverse sections be taken through the two 
above vertebrae, the ends of which are figured (figs. 18 and 21), 
the composition of the arches as well as the finer structure of the 
vertebrae may be made out. The neural arch in the second 
section (fig. 19) shows a great extent of the basidorsal cut and a 
decrease in the amount of the interdorsal. In the third section 
(fig. 20) the suprabasidorsal or neural spine (s.b.d.) also appears, 
so that all three of the parts making up the arch are seen here. 
In the caudal vertebrae the neural arch is similar to that just 
described. In figure 22 a bit of the suprabasidorsal is cut, and 
in both figures 22 and 23 the haemal arch shows the basiventral, 
that part of the interventral which appears in end view (fig. 21) 
not being touched. 
