TERMS APPLIED TO FINS. 41 



Pteracles trichipterus C\i\., as very long and remarkably 

 broad. 



(84.) The mimber of bays of which the several fins 

 are composed, affords one of the best characters for 

 specific distinction ; for although it has been said that 

 they vary in individuals of the same species^ we must 

 confess that this opinion has not been verified by our 

 own observations, — and they have neither been few, nor 

 partial, nor taken from preserved specimens. We are 

 more disposed to believe that such differences are more 

 apparent than real : first, because in many instances, 

 when the rays are very small and close together, or 

 very numerous, we have found it almost impossible to 

 attain perfect accuracy in this respect, except by re- 

 peating the examination several times, even on fresh 

 specimens ; and secondly, because the fins of many of 

 the ground-feeding families are so thick and fleshy that 

 the number of rays cannot be distinctly counted. The 

 eels, the Gadiadts, the SiluridfB, and several other 

 thick-finned families, are familiar instances of this ; but 

 very few will be found among the spine-rayed groups, 

 where, from the membrane being thin, the rays of the 

 dorsals, ventrals, anals, and even the pectorals, may be 

 numbered with accuracy. 



(44.) The external covering of the gills, and more 

 especially their aperture, are of great importance, and 

 require to be further noticed. The use of the gill-cover, 

 or operculum, is obviously to protect the gills themselves, 

 and, at the same time, to admit the egress of the water 

 taken in by the mouth : in the majority of typical 

 fishes it is moveable ; for, although composed of bony 

 plates, these plates are articulated or jointed at their 

 sutures by a membranaceous skin which acts as a hinge. 

 Properly speaking, the operculum consists only of three 

 pieces, which are attached to the cheek-bone, called the 

 pre-operculum {fig. 5. a): of these three plates, the upper 

 is more especially termed the operculum, and it is al- 

 ways the largest ; the next is the sub-operculum (b) ; 

 and the third, which is very small and sub- triangular. 



