280 OBSERVATIONS ON THE DEVELOPMENT OF 
chord. 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-ravs 
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, together with the elongated terminal centrum, is 
about 1-16th of an inch. The hypural apophyses are attached along 
the under surface of the ossified walls’ of the notochord. They are 
nearly equal in size, and each supports, as before, six rays, but the 
number in front of the anterior hypural apophyses has increased to 
SIX or seven. 
A short and rudimentary neural arch rises from the anterior end 
of the urostyle, and there is an indication of a second opposite the 
interval between the anterior and posterior hypural apophyses, where 
I have seen traces of what seemed to be a sutural division of the 
urostyle into two portions. 
The anterior epiural apophysis appears greatly enlarged and 
bifurcated at the extremity. I am inclined to think that its an- 
terior part represents the neural spine of the anterior neural 
arch of the urostyle, but it is separated from it by a wide in- 
terval. The posterior epiural apophysis is also enlarged and altered 
in form. 
In the adult fish (fig. 4) the urostyle is at once recognisable as 
a slender, tapering, bony process, in which an internal cavity can be 
observed, and which forms as great an angle with the axis of the 
vertebral column as before. The length of this process, together 
with that of the terminal centrum, of which it is a prolongation, 
is about I-14th of an inch, and no trace of the notochord is visible 
beyond it, so that I doubt not it is the result of the complete 
ossification of the walls of the chorda. The posterior hypural 
apophysis is as nearly as may be of the same size as the anterior, 
and, like the latter, carries six large fin-rays. These almost entirely 
support the tail, the fin-rays above the notochord not attaining 
more than one fourth their length, and constituting only a very 
insignificant portion of the root of the tail. The epiural apophyses 
are greatly altered, but I need not enter into a particular description of 
them. 
Thus it appears that Gasterosteus is in reality an excessively 
