10 



detail, in order to obtain a complete knowledge of their specific peculiarities. In 

 describing the characters of a fish, it is usual to commence by counting tho fin-rays in 

 each fin, the number of scales in the lateral line, wherever this is possible, the number 

 of vertebnc, and the number of rays whicli support the membrane covering the gills 

 beneath the operculum ; these are called the branchiostegal rays. These numbers 

 are all put down in succession with initial lettei's to indicate the organs they refer to, 

 so as to form a numerical foi'mula. Thus : — 



B. := Branchiostegal. 

 D. = Dorsal fin. 

 A. = Anal fin. 

 L.l. = Lateral line. 



Pt. = Pectoral fin. 

 Pv. = Pelvic fin. 

 C. = Caudal fin. 

 Vert. = Vertebras. 



The proportions of the fish are expressed numerically by stating the number of 

 times a given length is contained in the total length : the reason of this is that the 

 measurements are made with a pair of compasses, which are first adjusted to the length 

 of a certain organ, and then used to mark olT successive lengths equal to this along the 

 body-length. 



