NECTURUS MACULATUS. 399 
of which survives but two or three vertebrae, while the ventral one continues for two or 
three more. Pari passu with the loss of the transverse processes, the zygapophyses 
become reduced and finally disappear. 
With the loss of these lateral elements the 
vertebrae become narrowed from side to side, and, as this appearance is further increased 
by the great extension of neural and haemal arches and spines, the entire column becomes 
strongly compressed laterally, thus corresponding to the change in external form which is 
so apparent in the living animal. In this more characteristically caudal region (circa 
between 28 to 40) the hour glass shape of 
the centra becomes more apparent and 
in the reduction which follows towards 
the tip, this element becomes more and 
more conspicuous as the main portion of 
the vertebrae. Thus, after about the 
40th vertebra, rapid reductions take 
place in the arches, both neural and 
haemal, and the terminal one or two 
vertebrae consist of centra alone, the 
posterior end of the final one being 
rounded and without the characteristic 
cup. In this loss of arches it seems that 
the neural arch disappears first, as in all 
the cases examined several of the last 
vertebrae consisted of body and haemal 
arch alone, but the specimens examined 
were too few in number to establish this 
asalaw. From about the point at which 
the vertebrae first become laterally com- 
pressed, or more exactly, after the loss of 
the zygapophyses, there is shown an 
almost complete dorso-ventral bilateral 
symmetry, so that in an isolated vertebra 
it becomes extremely hard to distinguish 
between the neural and haemal arches. 
There is, however, a slight difference 
which, though difficult of formulation, 
may be perceived after a little study 
and seems to consist mainly in a some- 
(a) 
Fig. 5. Caudal vertebrae ; (a) and (b) two examples of ter- 
minal vertebrae ; (c) instances of double haemal spines ; (d) 
and (e) two successive vertebrae in the same series showing 
doubling of neural spines. The arrows in (d) pass through 
the neural and haemal canals. 
