STEUCTUEE AND DEVELOPMENT OF LEPIDOSTEUS. 
407 
An examination of the ventral part of the caudal fin in various Ganoids, Teleostei, 
and Elasmobranchii appears to us to show that there can he but little doubt that, in 
the majority of the members of these groups at any rate, and we believe in all, the 
same distinction between the ventral lobe of the caudal fin and the remaining unpaired 
fins is found as in Lepidosteus. 
In the case of most Elasmobranchii, a simple inspection of the caudal fin suffices to 
prove this, and the anatomical features involved in this fact have usually been recog¬ 
nised ; though, in the absence of embryological evidence, the legitimate conclusion has 
not always been drawn from them. 
The difference between the ventral lobe of the caudal fin and the other fins in the 
mode in which the fin-rays are supported is as obvious in Chondrostean Ganoids as it 
is in Elasmobranchii; it would appear also to hold good for Amia. Polypterus we 
have had no opportunity of examining, but if, as there is no reason to doubt, the figure 
of its skeleton given by Agassiz ('Poissons Eossiles’) is correct, there can be no ques¬ 
tion that the ventral lobe of the caudal fin is supported by the haemal arches, and not 
by interspinous bones. In Calamoicthys, the tail of which we have had an opportunity 
of dissecting through the kindness of Professor Parker, the fin-rays of the ventral 
lobe of the true caudal fin are undoubtedly supported by true haemal arches. 
There is no unanimity of opinion as to the nature of the elements supporting the 
fin-rays of the caudal fin of Teleostei. 
Huxley,"" in his paper on the development of the caudal fin of the Stickleback, 
holds that these elements are of the nature of interhsemal bones. He says (p. 39) : 
“The last of these rings lay just where the notochord began to bend up. It was 
slightly longer than the bony ring which preceded it, and instead of having its 
posterior margin parallel with the anterior, it sloped from above downwards and back¬ 
wards. Two short osseous plates, attached to the anterior part of the inferior surface 
of the penultimate ring, or rudimentary vertebral centrum, passed downwards and a 
little backwards, and abutted against a slender elongated mass of cartilage. Similar 
cartilaginous bodies occupy the same relation to corresponding plates of bone in the 
anterior vertebrae in the region of the anal fin; and it is here seen, that while the 
bony plates coalesce and form the inferior arches of the caudal vertebrae, the cartilagi¬ 
nous elements at their extremities become the interhaemal bones. The cartilage 
connected with the inferior arch of the penultimate centrum is therefore an c inter- 
haemal’ cartilage. The anterior part of the inferior surface of the terminal ossification 
likewise has its osseous inferior arch, but the direction of this is nearly vertical, and 
though it is connected below with an element which corresponds in position with the 
interhaemal cartilage, this cartilage is five or six times as large, and constitutes a 
broad vertical plate, longer than it is deep, and having its longest axis inclined 
downwards and backwards. 
* “ Observations on the Development of some parts of the Skeleton of Fishes.” Quart. Journ. Micr. 
Science, vol. vii., 1859. 
3 G 
MDCCCLXXXII. 
