36 



CLASSIFICATION OF FISHES. 



lobe of the tail is not formed, as in ordinary fishes, 

 by rays, but by the terminal vertebrae, round which 

 the fin is short; while the other, or lower lobe, often 

 irregular, is alone composed of rays. No other fishes, 

 yet discovered, possess this sort of caudal fin, nor is 

 there any thing analogous to it among the osseous or 

 semi-cartilaginous orders. Another modification of the 

 forked structure occurs in a very few genera, where 

 there are two divisions, or rather sinuosities, in the ter- 

 minal margin, analogous to the double fork seen in one 

 or two birds of the Caprimidgidce. Finally, this struc- 

 ture blends into the rounded form in such fish as have 

 the even tail already described, but with the middle rays 

 very slightly shorter than the outer ; so that the margin 

 becomes widely notched, or sinuated, as seen in several 

 of the salmon family, and many others. 



(37-) In some genera the caudal fin is either indis- 

 tinct or obsolete. The first appellation may be given, 

 when the fin is so united to the dorsal and ventral that 

 there is no perceptible difference between the rays of 

 either : such is the case in the greater number of the eels 

 and congers, in the genus Ophidium, in certain silures, 

 or cat-fish (Siluridce), and in several other anguilliform 

 types : in some these three fins form an acute point, as 

 in Ophidium, Synbranchus, &c; or a rounded one, as in 

 the lampreys (Petromyzon), and many of the soles, and 

 other Pleuronectidce. The caudal fin may be also termed 

 obsolete in most of the Raidce, where it either assumes 

 the form of one or two small lobes, or of merely a long 

 narrow membrane bordering the lower extremity of 

 the pointed filiform tail so common in this family. 

 The caudal, however, is completely obsolete in such ge- 

 nera as Trichiurus ; for in them the body terminates in 

 a long slender process resembling a filament. In Tri- 

 chiurus, Chimcera, and some Synynathi, the tail is desti- 

 tute of either a terminal or lateral fin ; and the same is 

 observed in many of the sting rays (Tryglince): but 

 in Gymnotus the under part of the tail is margined by 

 a continuation of the anal fin which reaches to the 



