MONTAGUS SEA-SNAIL. 



285 



was taken by Mr. (". A. Hansson off StrOmstad at a 

 depth of IK) more than a foot or so. 



In adult specimens in ^'eneral the Iciiiith of the 

 head is e(|ual to the gi-eatest de])th of tlic body, or less 

 than the latter, sometimes sinking' as low as 19 % of 

 tiK! length of the body. In the fry it is, as usnal, 

 I'elntively greater". The eyes are small, their longi- 

 tudinal diameter, in adult specimens fi-om 40 to 80 mm. 

 long, varying between 1 (i and 14 % of the Icngtli of 

 the head, oi- between 55 and 44 % of tlic breadth of 

 the slightly convex interorbital space, or between 55 

 and 75 % of the least depth of the tail. The bbnit and 

 in most cases, convex snout, the lengtii of which is 

 rathei' more than Va of that of the; head, is furnished 

 superiorly, in front of the eyes and somewhat nearei' 

 the middle of the head, with two tubular nostrils; and 

 behind the latter, just within the upper anterior corner 

 of the eye, there sometimes a])pears in young specimens 

 a small hole or transverse slit on each side, forming 

 the posterior nostril, but in most cases closed, covered 

 b\- tlie skin, and visible only as a. small dermal pro- 

 tuberance. The mouth, which lies at the very tip of 

 the snout, with the upper jaw- projecting only slightly 

 beyond the lower, is small, but fairly broad, the breadth 

 of the gajje, which is horizontal, measiu'ing about half 

 the lengtii of the liead. Tiie dental card on the inter- 

 maxillary bones, as well as that in the lower jaw, is 

 broad (containing many I'ows) in the middle, the teeth 

 being tricus])id, with the middle ])oint longest. The 

 lips, botli the uppei' and the lower, are each furnished 

 with a row of pores along the margin, generally con- 

 taining 4 pores on each side, and from the upper lip 

 thei'e also runs, in the direction of the nasal region, a 

 roAv of two similar pores on each side. The further 

 extension of the system of the lateral line (jn the head 

 is externally visilde in the poi'es round tiie eyes, and 

 in a row which is the only external indication of the 

 margin of the preoperculum ; l:)iit of the lateral line 

 proper we find onl\- two pores on each side, just above 

 the upper corner of the gill-o])ening. The latter is a 

 perpendicular slit, the lower third or fourth of which 

 lies just in front of the upper part of the base of the 

 pectoral fin. The length of this slit, however, varies 

 considerably, between */, and ^,'^ of the length of the 

 head. The anterior margin (jf the slit, or tiie oper- 



cular niai'gin, projects at about the middle into a more 

 or less obtuse dermal flap, wiiicii fits into the upi)er 

 angle of riic ])ectoral fin, and the thin frame of which 

 is formed by the end of the elongated, ti'iangular, forked 

 or almost crescent-sha]ied operculum, together with the 

 iiind part of the subjacent, narrow and bent suboper- 

 culum, wliicli is of almost uniform breadth. The rest 

 of tiic l)ranchiostegal membrane is united to tiie skin 

 of the bo(i\-, but forms a fold whicli hangs like a c(dlar 

 across the tiu'oat and the isthmus. This membrane is 

 often so tliin that one can easily count the six branchio- 

 stegal rays on each side of the body, togethei' witii tiie 

 interoperculum, wiiicii is of exactly the same shape, 

 and extends from tiie articulation of the lower jaw in 

 a, cur\e parallel to the lower posterior margin of the 

 ))reoperculum. The interoperculum has often been 

 i-eckoned I)}' mistake among the branchio.stegal rays. 



The pectoral fins are verv broad at the base, which 

 inferiorI\- cur\t's forward, in a direction ]jarallel to the 

 fold of the branchiostegal membrane and in front of 

 the ventral disk, the pectoral fin on the one side being 

 separated from that on the other by a distance no 

 greater than the diameter of the eye. All the rays of 

 these fins are simple and furnislied ^itii a fairly thick 

 skin, the lower ones being free at the tip. The lowest 

 rays are very short and only graduall}- increase in 

 length; but aliove the longest of them are a few slightly 

 shorter rays, the lowest part of each pectoral fin, with 

 the ti|)s of tiie raAs turned outwards, thus resembling 

 a distinct division or lobe of the fin. Tiie upper part 

 of the fin is obtusely rounded, the superior rays being 

 of fairly uniform length. The length of the fin from 

 the upper angle of the insertion is about 15 — 1772 ^ 

 of the length of the body, and from the front part of 

 the lower end of tiie liase about 24\!\~ 26 % thereof. 

 When folded the pectoral fins extend to the perpendi- 

 cular drawn just behind the vent, or even half-way 

 between tiic latter and the beginning of the anal fin. 

 The longitudinal diameter of the round or somewhat 

 oblong sucking-disk formed by the ventral fins, together 

 with its marginal fold, is about 11 or II ' ^ ^ of the 

 lens'tli of the bodv and ' ., the lenirtii of the head or 

 more. When the fish is in its normal state, this dia- 

 meter is onh' slightly less or even e(|ual to the distance 

 between the disk and the tip of the snout; but when 



" In a ninlc 71 iniu. long, from I'lolmslan, flie lengtii (if the head is only 1."' ., iiiiii.: in a young specimen 18 mm. long, from the 

 same lucalitv. the Icnsrth of the licad is 21 H 'i, of that of the liodv. 



