408 The American Naturalist. [May, 
believed by naturalists that the pectoral and ventral fins of the 
latter are the homologues respectively of the fore and hind limbs 
of the former. But between the paired fins of fishes and the 
median or vertical fins there are also resemblances which war- 
rant a suspicion of their origin from similar primitive structures, 
nsomuch that it is now believed by many that the median fins 
are derivatives from a primitive continuous vertical fold, while the 
paired fins are the specialized remainders of primitively continu- 
ous lateral folds. 
Cope, writing in 1870,° remarks upon the analogy that exists, 
on the one hand, between the succession in the type of the pec- 
toral fin, commencing with the simple axis of the Dipnoi, and 
passing to the axis with lateral appendages or radii, seen in 
Selache, and, on the other hand, the succession by which the 
median fins of Polypterus pass from rays with a jointed, forked 
extremity continuous with the base (in the caudal part of the 
series) to the structure that obtains in the dorsal pinnules, which 
consist of a strong spinous ray from the posterior face of which 
a number of soft rays project. He also regarded the segments 
which support the dorsal and anal rays in ancient fishes and 
Elasmobranchs as homologous with those which support the rays 
of the pectoral and ventral fins, and calls them all basilars, and, 
in a later essay, ° baseosts. He showed that the baseosts expe- 
rience the same reduction with time in all the fins, being obliter- 
ated in the dorsal, anal, and ventral fins in nearly all modern 
Teleostomata, and being reduced to four or fewer in the pectoral 
fin in most of the same. 
A most carefully worked out contribution to this portion of 
the developmental history of vertebrate appendages is that of J. 
K. Thacher,” who converts the analogy noticed by Cope into a 
homodynamism, and regards the Dipnoan paired fins, not as 
maintained by Gegenbaur,” as the primitive form or archiptery- 
18 Contribution to the Ichthyology of the Lesser Antilles, Amer. Philos. Society, 
— 446-483, 1870. 
19 AMERICAN NATURALIST, ci pP. 1016, 
20 Median and Paired dyes rans. Conn. Acad., Vol. III., 1877. 
"1 Jena. Zeitschrift, B Cie 4. Gegenbaur Lge den Gliedmaassen der Wirbel- 
omy im Allgemeinen rete das Hintergleidmaassen der Selachier in besonders. May, 


