640 



SCANDINAVIAN FISHES. 



togethei' form an elongaterl liorse-shoe, ecjual in It'iigth 

 to the sjjace between 'J of tlie free spinous rays on the 

 back. Within each arm of this horse-shoe projects a 

 small portion of tlie anterior extremity of one of the 

 pelvic bones. Tiic two pelvic l)ones are also elongated 

 and narrow in this species, their length being about 

 e(jual, in adult specimens, to the space between 10 or 

 even 11 of the free dorsal rays. Tiiey are thin and 

 almost flat, tlie inner sui-face alone being slightly hol- 

 low(;d into a l)road channel; and tliey are strengthened 

 along the middle by two ridges, an outer and strong 

 one and an inner and weaker, which is interrupted 

 at the middle. Above, towards the side of the body, 

 their thin margin is only slightly broadened at the 

 middle, and tliey do not send out any special branch 

 in this direction. Inwards, on the other hand, at the 

 lower ventral edge, at the middle of their length and 

 a little furtlier back, the)' send out a flat and thin 

 process, somewhat wider towards the tip, which meets 

 the corresponding process of the pelvic bone from 

 the other side of the body, though without entering into 

 any Arm osseous connexion with it. The pelvic bones 

 extend very nearly to a line with the vent; and here 

 this angular structure of the ventral edges ends, to be 

 replaced at the base of the anal fln and on the under 

 surface of the tail by a row of plates analogous to that 

 we have just seen on the dorsal side. The plates of 

 the lateral line, about 40 in numlx'r, fastigiated and 

 carinated, ^vith the carina; partly covering each other, 

 form a straight row from the temporal region above 

 the gill-cover out along the lateral edges of the com- 

 pressed tail, though here they grow thin and indistinct. 

 The first free spinous ray on the back is set above 

 the hind part of the gill-cover, at a distant'c from the 

 tip of the snout measuring between 24' ^ and 22 % 

 (generally decreasing with age) of the length of the 

 body. The last (hindmost) of these spinous rays lies 

 exactly opposite the vent, and the distance between it 

 and the tip of the snout varies individually between 

 45 and 46 % (exceptionally 43 %) of the length of the 

 body. Thus, during youth, the space occupied \yy these 

 spinous rays is, as a rule, somewhat less, in old spe- 



cimens somcM'hat greater, than the distance between the 

 first ray and the tip of the .-iiout. The length of the 

 rays is somewhat greater than the distance between 

 them; and their articulations lie a little to one side 

 and in a zigzag, so that, when they are depressed, tlie 

 ti|i of cacli ray falls beside the base of the next ray 

 l)(liind it, in the groove formed by the above-mentioned 

 concave i)lates". Ju.st behind the last spinous ray rises 

 the true doi'sal fln, triangular, with rounded ajiex, the 

 flrst ray being somewhat shorter than the second, but 

 equal in length to the third, the other rays decreasing 

 in length gradually, but sharply. Sometimes l)oth the 

 flrst and the second rays are simple, but in most cases 

 all the rays are more or less deeply branched. The 

 last ray is united throughout its length bv the fln- 

 membrane to the dorsal edge. When erected, the rays 

 are recurved, the anterior (as well as the posterior) 

 side of the triangle being thus convex. The leiigtli ot 

 the base of the fln is about Vs (varying lietween .j6 

 and 47 %) of that of the head. Just in front of the 

 anal fln we find a free s])inous ray'', equal in size 

 to the last spinous ray on the dorsal edge, but situated 

 further back, vertically below the first or even the 

 second ray of the true dorsal fin. Thus, the anal fin 

 being almost exactly similar to the dorsal in form, 

 structure, and size, the termination of the former lies 

 somewhat further back than that of the latter. The 

 form of the caudal fin reminds us, to a certain extent, 

 of a combination of the anal fin with the true dorsal, 

 in which combination each of these fins would cor- 

 respond to a half of the caudal fin, which is fan-shaped 

 when expanded. In this species, as in all the Scandi- 

 navian Sticklebacks, this fin contains 12 rays, excluding 

 a few (generally 2 or ?>) small supporting rays at each 

 margin), among which the 10 middle ones are branched 

 for about half their length, l)ut the outermost simple. 

 The middle rays of the caudal fin are alwaj's some- 

 what, though sometimes oidy slightly, shorter than the 

 longest rays of the dorsal fin, and measure about 8" 

 or 9 % of the length of the body. 



The jiectoral fins are of the form and structure 

 common to all the Sticklebacks. As a rule thev consist 



" When tlie numbiT of these r.ij-s is less than usual, it is generally the 14th that is wanting, as Kroyicu has leninrked, the distance 

 between the last ray and the last but one being tlius greater than that between the other pairs. The interspinal plate of the absent ray is 

 also wanting. In snt'h specimens we have also found the third spinous ray considerably reduced, and its interspinal plate proportionately 

 smaller than the others. In some cases Heincke found only 13 free spinous rays. 



■ In our figure this ray has unfortunately been overlooked. 



' Sometimes 7. 



