242 BRITISH FRESHWATER FISHES 



in contact ; when reduction takes place these three 

 plates (fifth to seventh) are the most persistent, 

 whilst the first to disappear are those which precede 

 the small keeled plates on the posterior part of the 

 tail. 



Some of the older writers recognized three distinct 

 species according to the development of the plates, 



(r. 



FIG. 22. Three-spined Sticklebacks, a. Rough- tailed (trachurus] 

 form. b. Smooth-tailed (gymnurtis] form. 



namely, the Rough-tailed Stickleback (Gastrosteus 

 trachurus] with a complete series of plates from 

 the head to the caudal fin, the Half-armed Stickle- 

 back (G. semiannatus) with a naked interspace 

 between the large anterior plates and the small 

 keeled plates of the caudal peduncle, and the 

 Smooth-tailed Stickleback (G. gymnurus) with the 

 body naked except for a few anterior plates. 



