THE FULL-ARMED STICKLEBACK. 193 



^ir Jenyns, who appears to have paid much attention to 

 the characters of these fish, says, that " G. leiurns, G. se- 

 miai matus, G. trachunis, and G. brachycentrus, are mere 

 varieties, subject to great variation, not only in the num- 

 ber of the lateral plates, but in several other less obvious 

 respects. The former may occasionally be found of every 

 intermediate number, down to that which characterizes G. 

 trachunis. This number, moreover, is sometimes found 

 constant in specimens whicl) differ remarkably in other re- 

 spects ; at" other times varying, when all other characters 

 remain the same."" Mr Yarrell states, that " he has taken 

 specimens of G. semiarmatus of all sizes, which were uni- 

 form in the number of lateral plates, and close examination 

 by a friend who has paid particular attention to this sub- 

 ject, has shewn that no point of ossification or induration is 

 to be found posterior to the last perfect lateral plate which 

 seldom passes beyond the line of the vent." 



Gastekosteus tkachukus.* — The Full-armed 

 Stickleback. 



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

 extending to the base of the tail. (Plate XXV.) 



Description. — From a specimen two inches and a half in length. 

 Body rather elongated of an oval form ; sides compressed ; scapular 

 plate, operculum, and preoperculum, rounded at the posterior bor- 

 der ; under jaw the longest ; dorsal and anal fins diminishing in height 

 from before backwards, and ending in the same line; tail fin slightly con- 

 cave at the end ; second dorsal spine the longest ; the third the short- 

 est, not half as long as the first ; lateral line commencing over the 

 base of the scapular plate, taking the curve of the line of the back 

 to the base of the tail ; first dorsal spine placed over the base of the 

 pectorals ; second over the ascending portion of the pelvic plate ; 



" Gasterosieus irachurus, Cuv., Yarr. G. aculeatus, Bloch, Don., Flem., 

 Jen. Full-armed Stickleback, Rough-tailed Sackleback, Banslickle, Sharp- 

 lin. 



VOL. VI I. N 



