288 



SCANDINAVIAN FISHES. 



which are above referred to, are sharply and easily distinguished by 

 the larger eyes, the more tumid snout, the longer pectoral fins, the 

 more anterior position (in most cases) of the ventral disk, and the 

 black peritoneum of Cydogaster Fabricii. But the transitions deprive 

 these characters of their value, and the dubiousness of these varieties 

 is forced upon us, especially when we have to decide to which of 

 them the Baltic form should be referred. We have already" declared 

 our opinion, when arranging the collection of fishes made by the Vega 

 Expedition in the Arctic Ocean oif Siberia, that Cydogaster Fabricii 

 is a variety of the Common Sea-Snail, and a new examination of all 

 the examples of this species belonging to the Royal Museum has still 

 further strengthened this impression. 



The Common Sea-Snail attains a consiflerably greater 

 size than the preceding speeies, but only in its true 

 home, the Arctic Ocean, where it may be at least 275 

 mm. long. In S^veden, however, it scarcely ever exceeds 

 half this size. The form of the body is essentially the 

 same as that of the preceding species; but tlie caudal 

 part is still more compressed, and its outlines converge 

 more sharply toAvai'ds the caudal fin, the least depth 

 of the tail varying between 3 and 4\'^ % (3' 1—4-2 %) 

 of the lengtli of the body. The head is generallj' larger 

 in this species than in the preceding one, its length in 

 most cases l)eing about V^ (from 24 to 27 %) of that 

 of the body''. The eyes are also larger, but vary con- 

 siderably in respect of size. They are, as usual, rela- 

 tively larger in young specimens than in old — in a 

 specimen 19 lum. long, from the Baltic, the longitu- 

 dinal diameter of the eye is 8'4 % of tlie length of the 

 body and Sl'.S % of the length of the head, in middle- 

 sized specimens (under 100 mm. in length) the usual 

 size of the eyes is 4 or h % of tlie former and from 

 15 to 20 % of the latter, while in the largest specimens 

 (170 mm. or more in length) it may sink to 2'/2 % of 

 the length of the body and 10 % of the length of the 

 head. We also find, however, that in the variety which 

 has received the name of Cydogaster Fahrlcli, the large 

 eyes are generally persistent, their longitudinal diameter 

 varying betAveen 6 and 8 % of the length of tlie liody 

 and 1 8 and 28 % of the length of the head. The breadth 

 of the interorbital space also varies considerably, be- 

 tween about 7 and 14 % of the length of the body, 

 but is generall}' greater than in the preceding species. 

 Tlie anterior pair of nostrils are rather high and tubu- 

 lar, the posterior generally appear merely as transverse 

 slits on each side, at the side of the anterior upper 

 corner of tlie eye, but the margin of these slits is also 



considerably raised in some instances. Among the jaw- 

 teeth, which \vith this exception rescMnble those of the 

 preceding species, Lutkex found, in old specimens, some 

 simple (single-pointed) teeth, like those which Stein- 

 DACHNER (1. c.) has described in the Californian species 

 Cijclofjaster (Liparis) pulchellus and Cydog. (Neoliparis) 

 mucosus. The character which lias been derived from 

 this peculiarity, and employed as a generic distinction 

 between this species and the following one, thus loses 

 its validity. The nature of the skin and the extension 

 of the system of the lateral line, on the head and the 

 body, are the saine as in the preceding species. Of 

 the lateral line on the body there generally appear, in 

 this species too, only the first two pores (ducts); but Lut- 

 kex has described perfect lateral lines, though with some 

 distance between the pores, in Cydogaster Fabricii, and 

 the lateral line is also complete in the largest specimens 

 of the common form from Spitzbergen. The small, 

 round, granular plates on the skin are more common, 

 especially on the head and the front part of the back, 

 in this species than in the preceding one. 



The pectoral fins are generalh* longer than in 

 Montagu's Sea-snail, especially in the Baltic form and 

 Cydogaster Fabricii, where their length from the upper 

 angle of the insertion to the tip of the longest I'ays 

 may rise to 20 — 25 % of that of the body. In young 

 specimens, however, in this species also, their length 

 is no more than 15 % of that of the bod}-. The dorsal 

 and anal fin are distinguished from those of the pre- 

 ceding species — apart from the greater number of the 

 rays, which has already been remarked — partly by 

 the fact that the descending curve in the margin of the 

 former is much less marked, sometimes quite imper- 

 ceptible, in this species, and that the longest rays of 

 these fins are situated somewhat farther forward, and 

 partlv by the fact that l)oth these fins are continued 

 much further out on the caudal fin, and in Arctic spe- 

 cimens Avhich are normal in other respects, are some- 

 times so closel)' united to the latter, that the line of 

 demarcation between them is externally invisible. We 

 find an example of this in Cydogaster Uneatiis as first 

 described by Lepechin; and the species thus departs 

 con.siderably from the form common in more .southern 

 localities, whicli in this respect sometimes comes very 

 near Montagu's Sea-Snail — so near that this character 



" Fish. Exhib., London 1883. 



* Only exceptionally have we found the length of the liead so small as 22 % of that of the body; and these exceptions may well be 

 due to the difficulty of taking sure measurements from tlic loose tip of the snout. 



