1890.] The Homologies of the Fins of Fishes. 409 
gium, but as a derivative from the primitive series of parallel 
rays of the lateral folds. 
- The conclusions of this writer respecting the median fins are 
as follows: The primordial median fin-rays are derived from a 
series of parallel chondroid rods, which grow up in the median 
fold totally independent of the cartilaginous arches above and 
= below the notochord. The earliest representatives of these parts, 
as the quadrate markings at the base of the median fin in 
Branchiostoma, and the fin-rays of Myxine and Petromyzon, are 
much more numerous than the vertebre opposite them, There- 
fore, neither are median fin-rays derived from neural spines, nor 
neural spines from primordial fin-rays, but neural spines are 
formed by the dorsad prolongation and union of the neura- 
pophyses of the vertebrz. 
This statement, which is supported by the structure, not only 
of the Bwer fishes mentioned above, but that of the Elasmo- 
branchs, which have from 2 to 3.5 fin-rays to one vertebra, and 
that of Acipenser, is in opposition to the earlier opinion of Gegen- 
baur,” who asserts that the primordial fin-rays, in their simplest 
form, are mere prolongations of the neural spines. 
The primitive fishes have been shown by Cope to sustain the 
view of Gegenbaur. The invariable equality of number between 
the vertebre and the actinophores (see section 3, below) in Dipnoi 
is supposed by Thacher to be the result of the entire loss of the 
primordial median fin-rays, the long and segmented supports of 
the median fold in Lepidosiren, Ceratodus, etc., being simply elong- 
ated neural spines. This view is not sustained by paleontology. 
Carrying his argument onwards to the lateral or paired fins, 
Mr. Thacher shows that the ventral and dorsal fins of Mustelus 
canis, agree in every respect save the separateness of the basal ele- 
ments in the latter, and their coalescence in the former, and argues 
that this certain amount of similarity warrants us in inferring 
an earlier state when the similarity was greater. The similarity 
between the structure of the anal and ventral fins of Acipenser is 
evident from an inspection of the figures. The theory put forward 
by Thacher, is, therefore, as follows: As the dorsal and anal fins 
22 Grundriss der Vergleichenden Anatomie, 1873, P. 488. 
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