406 
MESSRS. E. M. BALEOUR AND W. N. PARKER OK THE 
L. Agassiz that there is a complete parallelism between the embryonic development of 
the tail in these Fishes and the palaeontological development of this organ. We think 
that it is moreover convenient to retain the term homocercal for those types of caudal 
lin in which the dorsal lobe has atrophied so far as not to project beyond the ventral 
lobe. 
We have stated these now well-known facts to enable the reader to follow us in 
dealing with the comparison between the skeleton supporting the fin-rays of the ventral 
lobe of the caudal fin, and that supporting the fin-rays of the remaining unpaired fins. 
It has been shown that in L&pidosteus the unpaired fins fall into two categories, 
according to the nature of the skeletal parts supporting them. The fin-rays of the 
true ventral lobe of the caudal fin are supported by the spinous processes of certain of 
the haemal arches. The remaining unpaired fins, including the anal fin, are supported 
by the so-called interspinous bones, which are developed independently of the vertebral 
column and its arches. 
The question which first presents itself is, how far does this distinction hold good for 
other Fishes ? This question, though interesting, does not appear to have been greatly 
discussed by anatomists. Not unfrequently the skeletal supports of the ventral lobe 
of the caudal fin are assumed to be the same as those of the other fins. 
Davidoff,* for instance, in speaking of the unpaired fins of Elasmobranch embryos, 
says (p. 514) : “The cartilaginous rays of the dorsal fins agreed not only in number 
with the spinous processes (as indeed is also found in the caudal fin of the full-grown 
Dog-fish),” &c. 
Thacher,! again, in his memoir on the Median and Paired Fins, states at p. 284: 
“We shall here consider the skeleton of the dorsal and anal fins alone. That of the 
caudal fin has undergone peculiar modifications by the union of fin-rays with haemal 
spines.” 
MiyartJ goes into the question more fully. He points out (p. 471) that there is 
an essential difference between the dorsal and ventral parts of the caudal fin in 
Elasmobranchs, in that in the former the radials are more numerous than the vertebrae 
and unconformable to them, while in the latter they are equal in number to the 
vertebrae and continuous with them. “ This,” he goes on to say, “ seems to point to 
a difference in nature between the dorsal and ventral portions of the caudal fin, in at 
least most Elasmobranchs.” He further points out that Polyodon resembles Elasmo¬ 
branchs. As to Teleostei, he does not express himself decidedly except in the case of 
Mur<jena, to which we shall return. 
Miyart expresses himself as very doubtful as to the nature of the supports of the 
caudal fin, and thinks “ that the caudal fin of different kinds of Fishes may have arisen 
in different ways in different cases.” 
* Beitrage z. vergl. Anat. d. hinteren Gliedmassen d. Eische,” Morph. Jahrbnch, vol. v., 18/9. 
■f Trans, of the Connecticut Acad., vol. iii., 1877. 
1 St. George Miyart, “ Eins of Elasmobranchs.” Zool. Trans., vol. x. 
