190 FISHES OF THE FIRTH OF FORTH. 



Gasterosteus leiurus.* — The Quarter-armed 

 Stickleback. 



Specific Characters. — Back armed with three spines; lateral plates 

 not extending beyond the second dorsal spine. (See Plate 25.) 



Description. — From a specimen two inches in length. Body ra- 

 ther elongated of an oval form ; sides compressed; head granulated ; 

 cheeks smooth ; colour of the back and sides yellowish-brown ; tho- 

 rax and belly silvery- white ; male individuals., especially in , the 

 spawning season, are red under the throat and breast, and shaded 

 with bright green on the sides, but liable to great variation. Late- 

 ral line commencing over the scapular plate, and taking the curve of 

 the back to the base of the tail ; sides, over the pectorals, armed 

 with three or four scaly plates, which terminate under the second 

 dorsal spine, and not extending beyond the end of the pectoral rays ; 

 from the second dorsal spine to the tail, the sides are smooth with- 

 out scales, marked with transverse linear depressions, forming an angle 

 at the junction with the lateral line. Scapular plate and operculum 

 rounded ; preoperculum slightly produced at its inferior posterior 

 margin ; ascending portion of the pelvic plate passing behind the pec- 

 torals, and ending in two points a little under the lateral line. First 

 dorsal spine placed over the base of the pectorals ; the second, which 

 is the longest, over the last lateral plate ; the third, which is much 

 the smallest, over the termination of the pelvic plate. Dorsal fin com- 

 mencing close behind the last spine, and ending nearly in a line over 

 the termination of the anal fin ; tail fin slightly concave at the end ; 

 anal fin corresponding with the dorsal, but commencing farther back ; 

 a small bent spine in front of the first ray ; each ventral fin composed 

 of a strong serrated spine ; pectorals not reaching beyond the mem- 

 brane of the second dorsal spine ; eyes large ; jaws furnished with 

 small fine teeth ; none on the vomer or palatines ; under jaw the 

 longest ; base of the tail never keeled. Number of fin rays — 



D. 11 ; P. 10 ; V. 2 ; A. 8 ; C. 12 ; Branchial rays 3. 



It is distinguished from G. trachurus and G. semiarma- 

 tus by the lateral plates not extending beyond the second 

 dorsal spine ; but in what respect it differs from G. brachy- 

 centrus, both of equal length, I am not prepared to say. 



* Gasterosteus leiurus, Cuv., Yar. G. aculeatus, Penn. (p. Ixi.) Quar- 

 ter-armed Stickleback, Smooth-tailed Stickleback, Banstickle, Scotland. 



