3k HUXLEY, ON THE SKELETON OF FISHES. 
“This peculiarity, together with many other features 
characteristic of embryos, has naturally led me to examine 
into the relations which exist between these modifications 
and the characters which distinguish the fossil fishes of dif- 
ferent geological epochs. . . It is a fact well 
known to all anatomists, that the vertebral column of carti- 
laginous fishes does not terminate in the same way as that 
afi osseous fishes; in the former the bodies of the vertebrze 
become successively smaller from before backwards, and 
incline upwards more or less towards the end of the tail, so 
that the part of the vertebral column which carries the rays 
of the caudal fin forms a very open angle with the longitudi- 
nal axis of the trunk. A very peculiar form of the caudal 
fin results from this disposition : instead of being symmetri- 
cally bifurcated, it is simply bilobed, in such a manner that 
the superior lobe, situated, like the ‘inferior, under the pro- 
longation of the vertebral column, extends further back than 
the latter, which is produced only by an elongation of the 
anterior rays of this same inferior side of the vertebra. It 
results from this, that the caudal fin of the Plagiostomes has, 
properly speaking, no rays* inserted in the upper face of its 
vertebre. 
“In osseous fishes, on the other hand, the vertebral column 
terminates behind in a great expansion, whose superior 
and inferior apophyses are strongly dilated, so as to form a 
large vertical plate, whose posterior edge is symmetrically 
truncated, so as to present. an equal surface of attachment for 
the caudal fin-rays above and below the prolongation of the 
vertebral column. This caudal piece may be regarded as 
resulting from many vertebre soldered together, or else as a 
simple dilated vertebra carrying many yertical apophyses. 
The chorda dorsalis is continued in its interior, and is also a 
little 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 imto 
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 
* This statement, however, is incorrect, as Miller had long before shown. 
(Te He 
