320 PHILOSOPHY OF ZOOLOGY. 



employed exclusively in the construction of species, are de- 

 rived from their form, and the number of their rays. But 

 as these characters are liable to vary in different individuals 

 of the same species, they should be employed with great 

 caution. In many fishes there are numerous rays on each 

 side the different fins, so concealed under the skin that it is 

 impossible to count them, while others do not reach the ex- 

 tremity of the organ. Hence the number of rays must 

 vary with the mode of enumerating, and perhaps with the 

 age of the animal. The extent of variation occasioned by 

 the last cause, has not been satisfactorily determined. 



In expressing the number of rays in the different fins, it 

 is the practice of many naturalists to employ abbreviations. 

 Thus 1. D. 10. intimates that the first dorsal fin has ten 

 rays ; A. 10. that the anal fin has rays to the same amount. 

 When the fin consists of both soft and spinous rays, the 

 following symbol is employed, V . Here the ventral fin 

 has three spinous, and six soft rays. When there are spi- 

 nous rays on both sides of the soft rays of a fin, the cir- 

 cumstance is thus expressed, C 3 ^ 3 Here there are 

 three spinous rays on each side of twenty soft rays in the 

 caudal fin. These contractions are extremely useful in 

 shortening description, and are in common use. 



There are other abbreviations occasionally employed to 

 express the relative distance of the different fins. Suppos- 

 ing the length of a fish, from the extremity of the snout to 

 the origin of the caudal fin, to be twenty lines, and to its ter- 

 mination to be twenty-five lines, the result is thus contract- 

 ed, LC : A : : 20 : 25 ; the standard of comparison being 

 always the length from the snout to the beginning of the 

 tail. LC : DI : : 20 : 6, LC : DF : : 20 : 8, will intimate 

 that, while the length of the body is twenty, the length 

 from the snout to the beginning of the dorsal fin is six, and 



