STICKLEBACKS. 
85 
Poiss. to be insufficient to mark it with certainty as a species distinct 
from the full-armed Gasterosteus of Europe or America (p. 500). 
Dr. Parnell, in his Fishes of the Frith of Forth (p. 34), after stat- 
ing that he agrees with Cuvier and Yarrell in considering the G. trachurus 
as “ a constant and well-marked species,” observes that the “ square tail ” 
does not exist in the other sticklebacks. According to my observation, 
it is certainly less developed in them, and generally (but not invariably) 
corresponds with the protecting side-plates, presenting a greater or less 
development accordingly as the armature of the body is of a heavier or 
lighter cast. Dr. Parnell further remarks, as corroborative of G. trachurus 
being a distinct species, that he has “ examined carefully several hundred 
from half an inch to two inches and a half in length, and in all the speci- 
mens the lateral plates were constant.” In particular localities I have met 
with the same result on examining specimens of all sizes of G. trachurus 
and of the other varieties also,* but in some places again the different 
varieties are found together and of every size.f Mr. Yarrell has so 
noticed three of them in the Thames at Woolwich ; and in Rathlin, as 
before mentioned, they occur together — in the former locality in brackish, 
in the latter in fresh, water. 
G. semiarmatus , Cuv. and Val., t. iv. p. 493, appears to be the rarest of 
the 3-spined sticklebacks in Ireland. I possess specimens from the is- 
land of Rathlin, as before mentioned, and from Wolf hill, in the neigh- 
bourhood of Belfast. One example only occurred in the latter locality, 
where it was taken in 1832 with a number of G. brachycentrus, the stickle- 
back of that district ; it is, indeed, this variety in every respect, except in 
having the lateral plates extending along the sides so far as in G. semi- 
armatus ; the other characters assigned to this supposed species in the 
Hist, des Poiss. are very variable. From the half-armed species \ turn to 
the 
G. leiurus, Cuv. and Val., t. iv. p. 487, in which the lateral plates do not 
extend beyond the pectorial region. In every respect but this it is con- 
sidered in the Hist, des Poiss. so similar to G. trachurus , that the one 
description is given as equally applicable to both. The G. leiurus would 
seem to be the most common f reshwater stickleback in Ireland.^ 
The localities whence specimens of this fish are now before me, are — the 
island of Rathlin ; — the neighbourhood of Belfast (W. T.) ; — River Bann 
at Toome (W. T.) ; Portaferry and Newcastle, County Down (W. T.) ; — 
Lough Melvin, County Fermanagh (W. T.) ; — neighbourhood of Dublin 
(Dr. Ball) ; — Glendalough, County Wicklow (Mr. G. C. Hyndman) : — Port- 
arlington, Queen’s County (Rev. B. J. Clarke) ; — some of the examples from 
this locality are very handsomely marked, being along the back of a rich 
* The partial exception to this is in G. brachycentrus , in which the dorsal 
spines are comparatively longer in young than in adult individuals, and hence the 
young in this respect accord with G. leiurus. I here speak of localities in which 
all the full-grown fish are G. brachycentrus. 
f From the many small examples of all the varieties about nine lines in 
length that have come under my observation, I should think the number of 
lateral plates they are to possess through life is then as decided as the number of 
fin-rays ; i. e. provided they would have remained in the locality whence they 
were taken. Whether such a change of habitation, as from fresh water to the 
sea, would cause the smooth-sided at any age to put on the lateral armour may 
remain a question. 
X From drains which are occasionally replenished by the tide I have also 
taken it. 
