found in Ireland. 101 



is at that part rather flat, and is at least as high where the dorsal fin 

 originates as elsewhere. The Irish fish is in proportion to its depth 

 longer than the G. lietirus, as specimens of the latter under 2^ inches 

 in length, when compared with Irish specimens 3 inches long, proved 

 of equal dimensions (8 lines) at the deepest part. The difference is 

 also strongly marked in the relative breadth of the two species, the 

 Irish maintaining considerable breadth throughout, even to the ori- 

 gin of the caudal fin. The teeth in the lower jaw of the Irish spe- 

 cies consist in the centre of about four rows irregularly disposed, 

 but become gradually less numerous towards the back of the mouth, 

 where they terminate in a single line : the upper jaw contains three 

 rows in front, the outer and inner being regular in distribution. 

 In number, the G. lieurus which I examined does not possess so 

 many teeth as that species, but in their arrangement there is little 

 difference. On reckoning the vertebra in a specimen of the G. lieurus 

 and in one of the Irish Sticklebacks of similar length, I find that 

 the number in the latter exceeds that in the former species, and 

 that they are throughout more regularly equidistant than in the 

 G. lieurus. 



In the three English Sticklebacks, G. trachurus, G. semiarmatus 

 and G. lieurus, the bony plate covering the head is much stronger 

 than in the Irish fish — the outline of the lower jaw more angular — 

 the lips smaller and less fleshy — the number of rays in the fins dif- 

 ferent, consisting generally, in the Irish specimens, of twelve in the 

 dorsal, ten in the pectoral, eight in the anal, and twelve in the cau- 

 dal. In the three English Gasterostei, also, the ventral spine is longer, 

 but not so broad as in the Irish fish — the dorsal spines considerably 

 longer, and the plates whence they spring proportionately larger. 

 The following is the measurement of the spines in the four species : 



Total length of fish. First dorsal spine. Second. Ventral. 



G. trachurus 2 in. l|lin. S^^lin. 2| lin. 4 lin. 



G. semiarmatus ..26 2^ 3 4^ 



G. lieurus 2 6 2^ 3 4^ 



Irish species, \ 3 o U l-S S^ 



G. brachycentrus J 2 4 T 



In the last species* the membrane extends to the extremities of all 

 the spines. 



About Belfast I have taken the smooth-sided Sticklebacks 

 — G. lieurus and G. brachycentrus — from ditches in the low 

 grounds, from clear mountain-streams at an elevation of 600 

 feet above the level of the sea, from the muddy rivers Black- 

 water and Lagan, and from water which was partially salt 

 (here G. lieurus only), when, contrary to what might be ex- 



* Agi-eeably to the view taken in the ' Hist, des Poiss.,' the term "species" 

 was here applied to G. brachycentrus. I was disposed at the time (1S34) 

 to regard it as a local variety, but had not the means, which have since been 

 afforded by a comparison of specimens from numerous localities, to arrive at 

 a certain conclusion on the subject. 



