464 PBOF. ST. GEOEGE MIVAET ON THE 



Again, since in the Sharks the limbs are formed by the differentiation and predo- 

 minant growth of two portions of such an, at first, continuous fold, so, as different 

 species successively arose with different wants and requirements, different portions of 

 the fold may have been developed in different early forms ' with a resulting difference 

 of innervation in such differently arising paired limbs, the resemblance between such 

 diversely arising limbs being due to homoplasy. 



The results of my examinations appear to me to confirm the belief that the nature of 

 azygos and paired limbs is fundamentally the same. 



I suppose no one will dispute the truth of the doctrines: — (1) that when hard sup- 

 porting structures first appeared in the dorsal fin, such structures formed a longitudinal 

 series of similar, separate, more or less numerous parts — a condition they still present 

 in most existing fishes; (2) that more solid and complex structures in the dorsal fin 

 are secondary and derivative. 



These points being coricceded, have we evidence of actual coalescence in the skeleton 

 of the pectoral and dorsal fins ? 



Whatever view may be taken of the primitive pectoral fin (whether the archyptery- 

 gium be conceived like the limb of Ceratodus, of Lepidosiren, of Raia, or as having 

 arisen from divei-ging branchial rays), coalescence and segmentation must have taken 

 place to produce the existing Elasmobranch pectoral. 



As to the dorsal fin, we have incipient coalescence between radials in ScylUum cani- 

 cula (Plate LXXV. fig. 6) and Ginglymostoma cirratum (Plate LXXVI. fig. 2) ; and we 

 have found in Notidanus dnereus (Plate LXXV. fig. 2) a very remarkable instance of 

 such coalescence, most of the radials having come to repose upon one continuous basal 

 cartilage, an instance of coalescence equal to any thing found in the pectoral fin. 



Again, in Acantldas (Plate LXXVII. fig. 1), Spinax (Plate LXXVII. fig. 4), Callo- 

 rhynchus (Plate LXXIX. fig. 1), Rhyncobatus (Plate LXXVIII. fig. 5), Pristis (Plate 

 LXXVIII. fig. 4), and Pristiophorus (Plate LXXVII. fig. 6), as well as in Squatina 

 (Plate LXXVII. fig. 5), we have more or less striking examples of considerable, though 

 less complete or extensive, coalescence. 



But the pectoral fin has, as every one admits, the same essential nature as the ventral 

 fin ; and some ventral fins present a striking resemblance to dorsal fins. To see this it 

 is only necessary to compare the ventral fin of Chiloscyllium (Plate LXXVI. fig. 5) 

 with the dorsal fin of Raia (Plate LXXVIII. fig. 7), or the ventral of Notidanus 

 (Plate LXXV. fig. 4) with its dorsal (fig. 2). 



The anal fin may also be sometimes made use of to show this community of nature 

 between the azygos and paired fins. If the anal and ventral fins of Notidanus be 

 compared (Plate LXXV. figs. 5 and 6), it will be difficult to believe them to be of 

 radically different nature ; and a comparison between the anal and ventral fins of Poly- 



' This consideration has also been brought forward by Mr. Balfour, 1. c. p. 134. 



