HUXLEY, ON THE SKELETON OF FISHES. 41 
long and jointed at their ends, while the superior were simple 
styles, were connected with the posterior edge of the posterior 
or superior hypural apophysis. The terminal osseous ring 
(6) had in the meanwhile extended backwards, and now, as a 
slender tube, tapering posteriorly and obliquely truncated 
behind, embraced more than half the length of the previously 
free part of the notochord. As a consequence, the hypothe- 
nuse of the still triangular posterior hypural apophysis is 
now fixed to bone throughout its whole length, for the end 
of the bony sheath in question extends slightly beyond it. 
The remainder of the notochord (c) has its wall still mem- 
branous and unossified, and ends close to the superior and 
posterior angle of the caudal fin as before. There are no fin- 
rays above the notochord, nor is any neural arch developed 
from the terminal centrum, but the rudimentary interneural 
cartilage of the penultimate centrum had greatly elongated, 
and had taken the same position relatively to its superior 
arch as that occupied by the interhzmal cartilage relatively 
to the inferior arch, and had become surrounded by a sheath 
of bone. Behind this two other cartilages (m, n) lie parallel 
with one another above the ossified sheath of the chorda, but 
at present they are connected with no fin-rays. I will term 
these the “‘ epiural”’ apophyses. 
In a half-grown Stickleback (fig. 3) the anterior end of the 
terminal centrum was dilated and cup-lke, just as if it were 
the anterior half of one of the ordinary hour-glass-like ver- 
tebrze, but instead of dilating again posteriorly, it is continued 
into a stout style, more than twice as long as the body of the 
penultimate centrum, and curved up so as to make an angle 
of 45° with the rest of the vertebral column. ‘This stout style, 
with its central cavity, looks not very like the previous deli- 
cate sheath of the chorda; but such thickened sheath it really 
is, and with care the remainder of the notochord may be traced 
beyond it between two of the fin-rays into the tail-fin itself. 
The rays between which it lies are the uppermost of the su- 
perior set in the last-described embryo, and a new set, six in 
number, which have been formed above the notochord. I shall 
henceforward term this ossified chordal style the “ urostyle.” 
The free part of the notochord no longer reaches, by a long 
way, to the posterior superior angle of the caudal fin, for the 
fin-rays attached to the hypural apophyses, the uppermost of 
which supports the posterior superior angle of the caudal fin, 
are now more than twice as long as the free part of the 
notochord, and consequently the end of the latter is by its 
whole length distant from the present superior and posterior 
angle of the fin. The whole length of the free notochord, 
VOL, VII. E 
