24* CLASSIFICATION OF FISHES. 



composed of rays ; but these rays are so very slender, 

 and are so thickly covered by fat and skin, that in many 

 instances they cannot be counted without dissection : 

 the whole of the GadiadtF, in fact, have their dorsals 

 remarkably thick and fleshy, so that the rays by which 

 they are supported only become distinct towards their 

 termination. It is in this family, also, that we find a 

 modification of this fin unexampled among fishes. In 

 the rocklings, forming the sub- genus Motella, there is, 

 before the true dorsal, another, which may be termed 

 a rudimentary fin : it is composed of a great number of 

 extremely fine, short, fleshy filaments, resembling cirri, 

 preceded by one somewhat longer and thicker than the 

 rest, but all united at their base by a true membrane : 

 the pecuHarity consists in these filaments having the 

 form of rays, without the least degree of firmness; for 

 in other respects these fins are formed in the usual 

 manner, and are situated in a deep groove. 



(27.) The ^/^^(rfo?;^^?;^.?, or finlets, as they are sometimes 

 called, seen in mackerel and other allied genera {fig. 2. d), 

 may be considered as a modification of the true adipose 

 dorsals in the corresponding or analogous group of the 

 sahnons, among the soft-rayed families. They may be con- 

 sidered as single detached rays, excessively branched from 

 their insertion on the back, where they are remarkably 

 thick and fleshy : like the adipose fins before described, 

 ythe are always situated behind the first dorsal ; but 

 while no fish has yet been discovered with more than 

 one adipose fin, those which we are now speaking of 

 are almost always numerous, varying, among the mack- 

 erel, from four to seven, and even more. The only two 

 genera yet known, we beheve, where these finlets are 

 placed near to the head, are those of PoJypteriis and 

 Plesiops : in both these, indeed, they supply the place of 

 the true dorsal fin, the remnant of which, so to speak, 

 only shows itself in a few connected rays, adjoining and 

 unitins: with the caudal. 



(28.) The dorsals, as well as all the other fins, among 

 the cartilaginous fishes, are so thick, from being covered 



