VALUE OF TEETH AS CHARACTERS. 45 



remarkable are the variations in the teeth of M. Cuvier's 

 genera Pimelodus and Bagris : he himself observes this; 

 and yet these two groups are attempted to be charac- 

 terised by their teeth alone. Numerous other instances 

 might be named ; so that the only conclusion we can 

 come to is, that as no organs vary so much among 

 fishes as the teeth, so do they offer the most uncertain 

 characters, when taken by themselves, for designating 

 natural groups. For these reasons/ we consider such 

 characters inferior to those drawn from the fins, the 

 gills, the eyes, the body, and the scales. 



(46.) The LATERAL LINE, whcre it exists, as in the 

 more typical groups, deserves much attention : the 

 scales of which it is formed are always of a peculiar 

 construction, — being perforated in the middle for 

 the free issue of that mucous substance which is so 

 prevalent among fish, and which is secreted in certain 

 ^ands beneath : these scales are generally of a different 

 shape from those of the body; and they have been re- 

 cently employed by our best ichthyologists as additional 

 aids for discriminating species, which otherwise bear a 

 close resemblance. Sometimes, as in the family of 

 Scowibei'idce, the scales of the lateral line are raised and 

 carinated, so as to present a prominent edge like that 

 of the sharp ridge of a triangle ; while in others they 

 assume the form of spines or prickles : then, as to the 

 direction, it is either straight, arched, broken, or sinu- 

 ated. In some of the Indian SiluridcB it is double ; and 

 in many genera it cannot be distinguished. 



(47.) Many of the soft- finned fish are provided with 

 cirri, or barbels, placed round the mouth : these are 

 soft fleshy processes, and are supposed, with every ap- 

 pearance of reason, to be employed both as organs of 

 touch, and also of allurement to their prey. We con- 

 cur with Mr. Yarrell in believing that all cirrated fish 

 are ground -feeders, that is, seeking their food close to 

 the bottom. We may also remark, that such genera as 

 have these appendages very highly developed, as in 

 nearly all the SiluridcB, or cat-fish, they are employed 



