244 BRITISH FRESHWATER FISHES 



Sticklebacks ; thus typical examples of the gym- 

 nurus form occur in the rivers of Greenland. 



Towards the south the Three-spined Stickleback 

 is more generally a freshwater fish, and there is 

 reason to doubt whether it ever goes out to sea at 

 all in the Mediterranean. In our islands, as already 

 mentioned, a great variability is seen in specimens 

 from localities near the sea ; but when the lateral 

 series of plates is continuous (trachurus form) they 

 are not so deep and are often less numerous than in 

 northern examples, the caudal keel is weaker, the 

 naked area in front of the pectoral is smaller, the 

 pelvis is shorter and is notched in front, the fin- 

 spines are shorter or weaker, and the number of 

 fin-rays is usually less. Examples from Wands- 

 worth (gymnuruS) semiarmatus, and trachurus] differ 

 in no respect from others from the coast streams of 

 California. 



A nominal species, first described from Italy, is 

 the so-called Short-spined Stickleback (Gastrosteus 

 brachycentrus) ; this is only a gymnurus form with 

 very short fin-spines, and may often be met with 

 in inland localities in Ireland. I have compared 

 specimens of the gymnurus form from various parts 

 of the British Isles with others from Spain and 

 Italy, and from such far-away places as California 

 and Lake Biwa in Japan, without detecting any 

 differences. 



It is only when we get to the extreme southern 

 part of the range of the Three-spined Stickleback, 

 where it does not enter the sea, that we get fresh- 

 water colonies exhibiting peculiarities which may be 

 regarded as specific. Thus I have described a form 

 from Rome (G. hologymnus) in which the bony 



