420 Miscellaneous. 
to several families. In all the tail passes gradually from the condi- 
tion of a ventral appendage situated beneath the dorsal column, to 
that of a terminal tail placed in continuation of the line of the 
vertebre. The distinction between the two fins always occurs, but 
in a more or less distinct manner. 
The following are the consequences which the author draws from 
his observations :— 
Huxley and Van Beneden have opposed the theory of Agassiz and 
Vogt as to the parallelism existing between the embryonic tails of 
the existing osseous fishes and those of the fishes which appeared 
before the Jurassic epoch. Huxley appeals to the fact that the 
osseous fishes of the present day have tails the structure of which is 
really heterocercal. Van Beneden points out that in the Plagio- 
stomi the tail commences by being homocercal before becoming hete- 
rocercal, while, according to the theory of Agassiz and Vogt, the 
young Plagiostomi ought to have a preeminently heterocercal tail. 
A. Agassiz admits with Huxley that the form of tail called 
homocercal by Agassiz and Vogt is due only to a deceptive external 
appearance ; he also recognizes with Van Beneden that the young 
Plagiostomi, during the earliest periods of their development, have 
a strictly homocercal tail; but he nevertheless holds that neither 
of these two anatomists has upset the old theory of Agassiz and 
Vogt, and that we must retain this great generalization of the con- 
cordance of the embryonic with the paleontological development. 
It is only necessary to take another step, and to interpret somewhat 
differently from Agassiz and Vogt the arrangement of the tail 
which so predominates in the bony fishes of the present epoch. 
It must first of all be recongnized that heterocerceity does not 
correspond to the first stage, and that neither Von Baer nor Agassiz 
and Vogt have asserted that it does so, but that they have merely 
indicated this arrangement as characterizing one of the first stages 
of development. In point of fact, the fish, on issuing from the egg, 
has a nearly symmetrical tail, the notochord extending in a straight 
line in the direction of the caudal extremity. This stage, which 
represents the first form of the tail of the Teleostei as well as of the 
other groups, and which precedes that of the heterocercal tail pro- 
perly so called, is what A. Agassiz proposes to call the leptocardial 
state. 
Thus, from the embryogenic point of view, the tail of the Selachii 
is formed perfectly in accordance with the laws of development of 
the other fishes; and it only remains to see how far this agrees with 
paleontological history. 
If we examine the tails of the Devonian fishes we are struck 
with the exact parallelism that exists in this respect between these 
ancient representatives of the group and the successive stages of 
the tail in Pleuronectes. Among Devonian fishes there are some 
(e. g. Glyptolemus, Gyroptychius) of which the tail is distinctly 
leptocardial ; others (such as Holoptychius and Osteolepis) have 
slightly modified tail, presenting a very feeble tendency towards 
heterocerceity ; then come Glyptolepis &c., in which the hetero- 
