FINS OF ELASMOBRANCHS. 465 
pterus will teach the same lesson (Plate LXXIX. figs. 7 & 8). But perhaps the most 
instructive ventral fin is that of Polyodon (Plate LX XVIII. fig. 8), where the skeleton 
of this fin remains in its, as I suppose, primitive condition—a double longitudinal 
series of simple parallel radials, without enlargement at either end or coalescence. 
That is to say, it is in the condition of the simplest kind of dorsal fin as regards its 
cartilaginous supports. 
The ventral fin of Callorhynchus antarcticus is, as we have seen, very remarkable 
for another reason—namely, from its close resemblance to the pectoral. A close ea- 
ternal resemblance between these two fins appears in many Elasmobranchs; but here 
the resemblance is in the cartilaginous fin-supports, which is such as exists in no Shark 
or Ray known to me. 
This resemblance proves either (1) that both the pectoral and pelvic fins can be 
simultaneously and similarly modified, or (2) that a normal Elasmobranch ventral can 
assume the general appearance of a pectoral, or else (3) (if, as I am far from supposing, 
the ventral shows the original form) that the normal Elasmobranch pectoral has been 
changed in the overwhelming majority of cases from a condition more or less like that 
now found in the normal Elasmobranch ventral. 
If the ventral fin in the same great group can have the form either of a “dorsal” 
or of a “pectoral,” it is a strong argument that the “ pectoral” may also have at one 
time borne the aspect of a “ dorsal,” or of such a “ ventral” as that of Polyodon. 
One formidable objection, however, remains against the similarity in nature of 
dorsals and pectorals. It is that which is afforded by the fact that in most fishes the 
pectoral fins acquire a firm fixation to the axial system, through a shoulder-girdle, 
while the dorsal fins have no similar support. 
But we have seen that in Pristiophorus and Pristis (Plates LK XVII. and LXXVIIL.) 
the dorsal fin becomes directly continuous with the axial skeleton by a mass of car- 
tilage large enough to warrant comparison with the shoulder-girdle itself, while it is 
more or less firmly united to the axial skeleton or movably connected with it in a 
number of forms, e. g. Rhynchobatus, Squatina, Acanthias, Spinax, Chimera, and Callo- 
rhynchus. It must be conceded, however, that the direction of this attachment is 
continuous, direct, and longitudinal (7. ¢. antero-posterior), and therefore at right 
angles to the line of extension exhibited by the shoulder-girdle; and it may be asked, 
“Tf the pectoral fin is similar in nature to the dorsal, why, when it comes to contract 
adhesion to the axial system, does it not contract that adhesion by means of a con- 
tinuous, direct, and much extended antero-posterior connexion, as the dorsal fin does?” 
To this question it may be replied :— 
(1) That, whatever be the nature of the shoulder-girdle, whether composed of one 
arch or of several arches united, or of whatsoever other parts, it could not 
cohere continuously and antero-posteriorly with the axial skeleton without 
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