SOME PARTS OF THE SKELETON OF FISIIES 273 
bent upwards, so that, neglecting the osseous vertebral rings which 
surround the chord, it terminates as in the Plagiostomes. But the 
apophyses of this caudal piece are always disposed in such a way 
that those of the superior face carry the upper half of the rays 
of the caudal fin, and the inferior apophyses the inferior rays; and 
the result is a very regular disposition of the caudal fin, which is 
divided into two equal lobes, whose rays are inserted like a fan 
upon the spinous processes of the last vertebra, and arranged in 
such a manner that the rays of the upper lobe correspond to the 
upper apophyses, and those of the lower lobe to the lower apophyses. 
The slight differences of form and size which are sometimes remarked 
between the two lobes never affect the disposition of the rays ; 
for even when the caudal fin is cut square or rounded, it is not less 
invariably divided into two nearly equal parts, the superior of which 
is inserted on to the superior apophysis of the last vertebra. We 
may, then, regard this disposition as constant among osseous fishes, 
despite the slight inequality which is sometimes observed between 
the superior and inferior apophyses, and notwithstanding the curvature 
of the chorda at its posterior extremity.” 
M. Vogt then goes on to point out that since, according to 
M. Agassiz’s researches, all fossil fishes before the Jurassic epoch 
had inequilobed or heterocercal tails, while those with equilobed or 
homocercal tails only appeared subsequently, there is a parallelism 
in this respect between the several stages of the embryo of such 
a (Cycloid) fish as a Coregonus, and the groups of fishes which have 
at successive epochs peopled the waters of the globe. In his ‘ Re- 
cherches sur les Poissons fossiles, vol. ii, p. 102, the same doctrine 
is thus concisely expressed by M. Agassiz: 
“On the other hand, there is neither in the actual creation nor 
in anterior epochs, any adult fish belonging to these two last orders 
(Ctenoids and Cycloids) which has the vertebral column bent up, 
and the caudal fin inserted below it; whilst this arrangement is 
characteristic of embryos in a certain period of their existence. 
There is then, as we have said above, a certain analogy, or rather 
a parallelism, to be established between the embryological develop- 
ment of the Cycloids and Ctenoids, and the genetic or palzontological 
development of the whole class.” 
Professor Owen (‘Lectures on Fishes,’ 1846) describes the caudal 
fin of the ordinary osseous fishes thus: 
“The framework of the caudal fin is composed of the same 
intercalary and dermal spines superadded to the proper neural and 
heemal spines of those caudal vertebrae which have coalesced and been 
VOL. II T 
