Bashford Dean 215 
of and in the neighborhood of the dorsal fins. They are distinctly sep- 
arate from the neural arches and are apparently metameral ; their interest 
is obvious from the standpoint of fin morphology, for they can be in- 
terpreted as the rudiments of the basalia of a more continuous type of 
unpaired fin. 
Fins and Girdles.—The fins of Acanthodians are beyond peradventure 
of a lateral fin-fold type, and as such they have been given a 
prominent place in the much-discussed problem of the origin of the 
vertebrate limbs. From this standpoint it is clear, either that 
the lmb structure of Acanthodians and other primitive sharks 
must be reducible to a common. plan, or that the curious spine 
supported Acanthodian webs must have arisen sui generis. The latter 
view is difficult to accept, since it is conceded unanimously that the 
paired limbs of all other vertebrates are homologous, and it seems, 
therefore, if only from purely @ priori grounds, illogical to assume 
that Acanthodians, which are sharks in so many details of structure, 
could not have had fins based essentially upon shark-like structures. 
The view, moreover, of the homology of the fins of Acanthodians and 
sharks “ is supported by the evidence of Cladoselachian sharks whose fin 
structures are in important regards intermediate in type. In both forms 
the paired fins functioned as balancing organs, rather than as paddles, 
and in Cladoselachians there is a concentration of the supporting ele- 
ments, radials, in the anterior rim of the fin which, I have maintained, 
served as the ancestral condition of the spine of the Acanthodian.” This 
view is supported by the following facts: The caudal fin of Acantho- 
dians shows the radials in the process of concentration in the anterior 
rim of the fin. There is no spine present, although the anterior fin 
margin is encrusted and stiffened by shagreen. Such a condition has 
already been noted by various writers; it is admirably seen in one of 
Professor Jaekel’s beautifully prepared specimens of Acanthodes bronna, 
which I have figured herewith (Fig. 25). And similar conditions are 
known in other genera. In Parexus falcatus,” for example, the hypural 
lobe of the caudal fin is strengthened by shagreen to such a degree as to 
14 Cf. 1894, Jour. Morph., Vol. IX, pp. 98-111; 1896, Anat. Anz., Vol. XI, pp. 
677-679; 1896, Nat. Science, Vol. VIII, pp. 245-253. 
4% In this evolution the dermal elements played an important part, encrust- 
ing and strengthening the anterior rim of the fin, a process which caused or 
was accompanied by a reduction in the radials. 
1 British Museum, P. 130. 
