192 FISHES OP THE FIRTH OF FORTH. 



Gasterosteus semiarmatus.* — The Half-armed 

 Stickleback. 



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

 not extending beyond the line of the vent. (See Plate XXV.) 



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

 ther elongated of an oval form ; scapular plate, operculum, and pre- 

 operculum, rounded at their posterior margin ; under jaw the long- 

 est ; dorsal and anal fins gradually diminishing in height from the an- 

 terior rays ; caudal fin even at the end, or very slightly concave ; 

 second dorsal spine the longest ; the third very small, not half the 

 size of the first. Colour of the head, back, and sides, yellowish- 

 brown, with a shade of green ; cheeks, thorax, and belly, silvery- 

 white. Lateral line commencing over the upper part of the opercu- 

 lum, following the curve of the back, and terminates at the base of the 

 tail ; sides armed by a number of scaly plates, which do not extend 

 beyond the line of the vent, from thence to the tail the sides are nak- 

 ed, marked with transverse linear depressions, forming an angle at 

 their junction with the lateral line. First dorsal spine placed over 

 the base of the pectorals ; the second over the ventral spine ; the 

 third over the termination of the pelvic plate. Dorsal fin commenc- 

 ing close behind the last dorsal spine; anal fin commencing behind 

 the vent, and both fins terminating in the same line ; each ventral fin 

 composed of a strong serrated spine, commencing at the base of the as- 

 cending portion of the pelvic plate ; pectorals small, even at the end ; 

 eyes large ; teeth fine and sharp in both jaws ; none of the vomer or 

 palatines ; base of the tail smooth, never keeled ; a small curved 

 spine at the base of the first anal ray. Number of fin rays — 



D. 10 j P. 10 ; V. 2 ; A. 9 ; C. 12 ; Branchial rays 3. 



This fish is of much less frequent occurrence than G. 

 biurus, or G. trachurus, although found to inhabit the same 

 places. It seldom exceeds two inches and a half in length, 

 and is supposed by Jenyns to be only a variety of the leiu- 

 rus. They certainly are very much alike, but the fact of 

 the lateral plates extending beyond the end of the pecto- 

 rals and not passing the vent, is considered by Cuvier and 

 Yarrell as a sufficient character to constitute it a distinct 

 species. It is found occasionally in the marshes below 

 Kincardine, and in the ditches in Guillon Links. 



* Gasterosteus semiarmatus, Cuv., Yarr. 



