PIKS OP ELASMOBEANCHS. 465 



pterus will teach the same lesson (Plate LXXIX. figs. 7 «& 8). But perhaps the most 

 instructive ventral fin is that of Folijodon (Plate LXXVIII. 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 ^.r- 

 ternal resemblance between these two fins appears in many Elasmobranchs ; but lieie 

 the resemblance is in the cartilaginous fin-supports, which is such as exists in no Shark 

 or Eay 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 Pristio])lionis and Pristis (Plates LXXVII. and LXXVIII.) 

 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, Spinas, Chinuera, and Callo- 

 rhynchus. It must be conceded, however, that the direction of this attachment is 

 continuous, direct, and longitudinal {i. e. antero-posterior), and therefore at right 

 angles to the line of extension exhibited by the shoulder-girdle ; and it may be asked, 

 " If 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 coimexion, as the dorsal fin doesi" 



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 



3r2 



