42 HUXLEY, ON THE SKELETON OF FISHES. 
together with the elongated terminal centrum, is about 1-16th 
of aninch. 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 apophysis 
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. Iam inclined to think that 
its anterior part represents the neural spine of the anterior 
neural arch of the urostyle, but it is separated from it by 
a wide interval. 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 1-l4th 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 sup- 
port 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 
heterocercal fish, the whole of its principal fin-rays being 
developed below the vertebral column. It is as heterocercal 
as an Accipenser, and far more so than a Scyllium ora 
Squatina. Furthermore it appears that the tail of this 
Acanthopteran fish has essentially the same structure as that 
of the Malacopteran salmon, except that the wall of the 
notochord is ossified through its whole extent, whereas in the 
salmon it persists in the condition which it has in the young 
Gasterosteus. I have not been able as yet to obtain so com- 
plete a series of forms of the caudal extremity in the Eel, 
but with some extremely interesting minor variations, which 
I propose to describe at length on a future occasion, the 
