i84 OUTLINES OF ICHTHYOLOGY. 



the tail fin, above and below the longest rays, and these are 

 frequently omitted from the reckoning ; when they are men- 

 tioned, it is in general in the shape of a fraction ])laced after the 

 number of the full-length rays, the numerator denoting those 

 above the fin, and the denominator those below it, as in the 

 example. When there is more than one dorsal fin, the rays are 

 enumerated with a line between them — • 



D. 9 — 12 : 

 the first numeral having reference to the first, or more forward, 

 fin. 



In the fish having two dorsals, as the Perch, the first is 

 usually wholly spinous, and this is indicated as in the last case, 

 by a line between the two numbers ; when, however, there is 

 but one dorsal, of which the fore part only is spinous, the sign 

 + is placed between the spinous and soft rays. 



This is also the rule in regard to other fins partly spiny and 

 partly soft : thus — 



(Fin-rays of Perch.) D. 15 — 1 + 13: P. 14: V. 1+5: 

 A. 2+8: C. 17. 



Some of the soft-finned fish have an occasional spinous ray ; 

 and when this is the case their position, &c., should be fully 

 described — a perpendicular line after such spinous ray being 

 drawn in the ray formulary. Take the fin-rays of the Common 

 Carp for example : — 



D. 4/19: P. 17: V. 9: A. 3/5: C. 19. 

 In this instance the fourth dorsal and third anal fin-rays are 

 spinous. 



T\^>i form of the gill-covers has been mentioned 

 as one of the points to be relied on for distin- 

 guishing one species from another ; for whilst, 

 owing to circumstances of food, water, or climate, 

 different individuals of the same species will often 

 differ widely from each other, and from their ori- 

 ginal type (in so far at least as regards those ex- 

 ternals which most readily strike an unscientific 



