SOME PARTS OF THE SKELETON OF FISHES 281 
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 Sepé/ium or a Sguatina. 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. 1 have not been able as yet to obtain so complete 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 structure is similar in principle. The 
tail is truly heterocercal. What answers to the urostyle is divided 
into two portions—the anterior of which supports the anterior 
hypural apophysis, the posterior the posterior; and the last is not 
only superior to the anterior hypural apophysis, as is the case in 
the Gasterosteus, but projects beyond it posteriorly. Seeing the 
close resemblance in the structure of the tail which exists among 
all Acanthopteri—inasmuch as the hypural apophyses resemble more 
or less closely those of the stickleback, and always bear the principal 
caudal fin-rays, 1 make no doubt that what is true for Gasterosteus 
is true for all, and by a parity of reasoning, that what is true for 
Anguilla and Salmo is good for all Alalacopteri ; and 1 therefore do 
not hesitate to draw the conclusion that the Ctexozde? and Cyclotdet 
of M. Agassiz, so far from being homocercal, are in truth excessively 
heterocercal ; that is to say, more completely heterocercal than the 
great majority of Elasmobranchi. 
In the heterocercal tails of the Ze/eostec there are, however, at 
least two well-marked varieties or grades of structure—the one, which 
might be called gymnochord tails, having the end of the notochord 
unprotected by ossification in its wall, as in the Stegurz of Heckel ; 
the other, which might be termed steganochord, having the end of 
the notochord enveloped in a styliform osseous coat, which there 
seems reason to believe represents the centra of two vertebrae. 
As a common, if not universal, character of the Teleostean hetero- 
cercal tail, by which it is distinguished from that of Elasmobranchiz, 
we have the peculiar development of the epiural and hypural 
apophyses. 
But if it be true that all Ctenoids and Cycloids are heterocercal, 
it is clear that the ground of the argument of MM. Agassiz and Vogt 
is completely cut away, so far as mere heterocercality goes. The 
ancient and the modern fishes are precisely on the same footing, and 
if the paleozoic Ganoided and Elasmobranchi really represented 
