?2 FIN-RAYS IN CLASSIFICATION OF FISHES. 



uiipairod fins. It follows, therefore, tbat the superimposed rows of 

 actinotrichia found in the fins of many of the Sharks is ;ni advance 

 upon the primitive single-rowed arrangement seen in Dipnoi, Holoce- 

 phali, and Teleostei. There have, therefore, been two lines or routes of 

 specialization in the development of actinotrichia, viz, (1) that charac- 

 teristic of Elasmobranchia and (2) that distinctive of Teleostei. 



Only amongst the Rays and Skates do we find an approximation to 

 the arrangement met with in Dipnoi and embryo Teleostei. In the 

 Eays, however, the actinotrichia are quite rudimentary — embryonic — 

 in the paired fins, so much so that they are confined to a very narrow 

 marginal portion of the pectoral, for example, not over an eighth of an 

 inch in width in specimens a little over a foot in length. This short- 

 ness and rudimentary condition of the actinotrichia in the paired fins 

 of the Eays is correlated with the great length of the actinophores or 

 cartilaginous rays supported by the pro-, meso-, and metapterygium, 

 themselves formed by the fusion of the proximal ends of actinophores. 



The only fins found in the Teleostei which retain the primitive feat- 

 ures of the continuous ones of the Dipnoi are the so called " adipose 

 fins" of Salmonoids, Nematognaths, Characinids, &c., but in them a 

 primitive structure is retained by the posterior dorsal only. But these 

 "adipose fins" are part of a discontinuous system of vertical fins, a 

 portion of which is developed to the degree characteristic of Teleostei 

 with interradial spaces. The adipose fins represent, in fact, the survival 

 of a Dipnoiln character as a part of a Teleostean organization. 



The theory according to which such a survival was brought about 

 seems to be the following: Inasmuch as "adipose fins" are embryonic 

 in structure, just in the same way as the radii of the fins of Dipnoi are 

 permanently embryonic, we are forced to infer that such tins, co-exist- 

 ing as they do with others in the same fish, having well developed 

 membranous, radial interspaces and ossified rays, have been retarded 

 in development so as to retain embryonic characters. The degenera- 

 tion, or rather retardation, of development of the second dorsal, which 

 is apparent in the Salmonoids, has been completed in the Cyprinoids, a 

 group which has entirely lost the posterior soft dorsal, retaining only 

 the anterior dorsal, with bony rays of the Teleostean type. The 

 Cyprinoid series has, however, acquired other structural specializations, 

 such as th« development of a system of auditory ossicles, coincidently 

 with which the anterior portion of the vertebral column has been modi- 

 fied. Turning now to the Nematognaths, a majority of them have tc- 

 tained the " adipose" second dorsal, supported by actinotrichia, while 

 they have acquired two new structures not met with in the less modi- 

 fied and older Salmouoid organization, viz, the system of maxillary, 

 mental, and nasal barbels, supported even in the embryo by cartilage, 

 and ossicula auditus, and often a peculiarly modified air-bladder, bi- 

 furcate anteriorly and coming into close contact laterally and an- 

 teriorly with the skin in the fore i)art of the body-cavity, so as to ap- 



