240 ACANTHOPTERYGII. 



four-spined stickleback, which he would not but look on otherwise than merely 

 an accidental variety of G. aculeatus, Linn. It was among a parcel consisting of 

 G. gymnurus, G. branchy centrus, and G. pungitius, taken in a pond and in some 

 neighbouring drains. The ascending plate from the base of the ventrals he 

 found to be subject to variety in form like other parts. While that the fish 

 under consideration was G. spinulosus he did not think admitted of a doubt. 

 Allman records the same (N. H. Ireland, iv, p. 89). Johnston (Berwick. Nat. 

 Club, 1838, p. 171) likewise considered this form a mere variety. At Edge- 

 worthstown I obtained this fish under the identical conditions as Thompson, and 

 came to the same conclusions. It appeared as if a dorsal ray had taken on a 

 spinous character, each having D. S/-^, whereas the common type had D. 2/-^. 



B. iii, D. 2-3(4)^1^, P. 10 (-12), V. 1/1, A. ¥ 2 ¥ , C. 12, Coecal pyl. 1 (2), 



Vert. 15/16. 



Length of head 3| to 4|, of caudal fin 7j to 8, height of body 3| to 4 in the 

 total length. Eye — diameter 3-| to 3| in the length of the head, 1 to 1 §- diameters 

 from the end of the snout, and 3/4 to 1 apart. Sides of the body compressed. 

 Snout of moderate extent : the cleft of the mouth oblique, the lower jaw slightly 

 in advance of the upper, the posterior extremity of the maxilla reaching to 

 beneath the posterior nostril. Nostrils midway between the eye and the end of 

 the snout. Opercles smooth : interorbitals wide. Teeth — fine, those in the upper 

 jaw anteriorly in three rows, decreasing to one posteriorly : in the mandibles in 

 four irregular rows anteriorly, decreasing in size posteriorly where they become a 

 single row. None on the palate or tongue. Fins — the first dorsal consists of 

 free spines from two to four in number, the first commences above the base of 

 the pectoral fin, while their height is subject to considerable variations. These 

 spines may be serrated or even denticulated on their outer edges, and each 

 articulates at its base with an osseous plate, while posteriorly they are connected 

 to the back by a short membrane. Pectoral attached to a broad, flat scapular 

 plate of varying size. Ventral consisting of one spine of inconstant length, and 

 either serrated or smooth, it is attached to the pubic plate, a bony, arrow-headed 

 expansion of the humeral arch which is subject to variation in its width and 

 length. A bony plate ascends from the pubic plate to the lateral scutes. Anal 

 situated beneath the last half of the dorsal : caudal with its posterior extremity 

 square, or rather emarginated. The bony plates along the sides vary considerably 

 in extent, some occupying almost the entire depth of the side, others much less : 

 their numbers likewise vary, as already pointed out, under the head of varieties ; 

 they commence behind the occiput and may only be from two (or even none, 

 according to Canestrini) in number, up to thirty-five or even forty. Above 

 these plates are others passing upwards to the back. The situation of the lateral- 

 line likewise varies, being nearer the dorsal profile in some varieties than it is in 

 others. Ccecal appendages — a rudimentary one in Gloucestershire examples. 

 Colours — in the marine variety the back is of a steel gray, having greenish 

 reflections, becoming golden on the sides and silvery beneath, the fins yellow 

 with fine black dots. In the fresh waters the back is more of a brown gray with 

 golden reflections, and a series of bright spots or vertical bands along the 

 body. During the breeding season the lower part of the head and chest and 

 under surface of the body becomes of a bright pink or red. The females are not 

 so bright coloured as the males. Thompson remarked that full grown fishes, in 

 which the most intense shade of red prevailed, never appear to be with spawn, 

 very few in that state being so much as faintly tinged with it. 



Varieties. — In structure these consist in (1) variations in the number of spines 

 forming the first dorsal fin ; and (2) variations in the number of bony plates 

 with which the sides are armed. In April or May, 1864, a five-spined stickleback 

 was captured in a stream near Warrington (J. Pears, Zool. p. 9145). Whether, 

 suggests Thompson, a change of habitation, as from fresh water to the sea, would 

 cause the smoothsided at any age to put on the lateral armour, may remain a 

 question. In colour — Thompson observes of the varieties brachycentrus and 

 gymnurus, that of many of the larger individuals captured in Ireland during the 



