AKCTIC I-LOINDERS. 



405 



Horii — \vith tlie siirt'ju'e of tlic 1)()(1\- ;iliiu)st entirely 

 smooth, with the exception of tlie weak spinous warts 

 at the spots mentioned and thinly scattered on the head 

 — lead us to the following conclusion. The character 

 drawn from the covering of scales, which gives us one 

 of the most important external distinctions, has deve- 

 loped from an original type common to \n)th species. 

 We are apparently guided to this original type l)y a 

 comparison between these two species and a third, 

 /'IritrtiiH'i-ft's ffluchd'i^, wiiich has a wide geographical 

 range in the North, and is as variable as either of the 

 others. This noi'tiiern species lias also borne many 

 names: I'leuroiiectes (jlacialis" , PI. cicafricosii^'', I'lafessa 

 fllahra'', Flafessa dvinensis'', I'leitronccfcs fr(iHldmii' and 

 EKchalamdiis putnami'. The first two names refer to 

 the two distinct varieties (figs. IIU and 111) wliich 

 I'allas knew from the European and Siberian coasts 

 of the Arctic Ocean; the third, the fourth, and the last 

 names Avere conferred on the species on the Atlantic 

 coast of North America — where it has Ijeen found as 

 far south as Salem — and on the Arctic coast of Eu- 

 rope. The last name but one lias reference to the spe- 

 cies as it occurs on the Arctic coast of North America. 

 In Greenland and Iceland, up to the present at least, 

 the species has not Ijeen found. 



Pleuronectes glaciaJis is distinguished l)oth from the 

 Plaice and the Elounder In- the greater de])th of the tail, 

 the least depth of which is ahvays more than y % of the 

 length of the body — a measurement we have found in 

 the Plaice only exceptionally, and ne\er cpiite exactly, 

 though very nearly so. This greater depth of the tail 

 gives to FI. (fJaclaUs the same character as I'J. liiiianda 

 acquires bv the comparatively small size of the head, 

 the least depth of the tail being more than 36 % of 

 the length of the head; while in /'/. jthifrssa and /'/. 

 r/esns this proportion is ahvays — to the l)est of our 

 knowk'dge with lOily one single exception — less than 

 35 %. Another character, which on the other hand 

 reallv ranges PI. glacudis in immediate systematic prox- 

 imit\- to I'l. limanda, consists in the scales of the body. 

 !))■ far the greater part of the eve side, or at least 

 those pai'ts where the scalv covering is strongest in the 



two preceding species, are clothed with ciliated scales, 

 in type the same as those of PI. limanda, but shorter 

 (smaller) and scattered, never imbricated, but I'ather set 

 in a (juincnnx; and as a rule, one row (jf these scales 

 advances at least over the middle rays of the dorsal 

 and anal lins. As usual, however, the scales m.ay also 

 var}- in /'/. /jhicialis; and sometimes the ciliated scales 

 occur only on the eye side, in a i-ow along the bases 

 of the dorsal and anal tins, while they are sometimes 

 close-set and contiguous, but not ind)rieate(l, on the 

 whole of the eye side. On the blind side they may 

 be entirely wanting or well-developed and only slightly 

 more scattered than on the eye side. In the number of 

 the vertebra?, too, PI. glacialis comes nearest PI. lliiKtinla. 

 According to our ol)servations the former possesses 3S 

 or 39 (according to Richardson 40) vertebne, while 

 J'l. liitKiiida has 39 or 40. In this respect, therefore, 

 both /'/. f/lacialis and J'l. linxn/dii appear as inter- 

 mediate forms between PI. y/r.s-«.s', with 35 — 37 vertebra-, 

 and /'/. pidtcssa, with 43 or 44. By the shajx' of the 

 lower pharyngeals and the structure of their teeth, how- 

 ever, I'l. f/lacialis is ranged in the same group as I'l. 

 ffesiis and I'l. platessa; and in this respect it comes 

 nearest the Plaice, in which these bones are narrower 

 than in the Flounder. I'l. f/lacialis also comes nearest 

 PI. plalessa in the mnnl)er of gill-rakers, which varies 

 on the first branchial arch between 9 and li'. In the 

 cephalic characters /'/. f/lacialis is approximated most 

 nearly to the Flounder by the bony ridge behind the 

 eyes, which is continuous or broken up into only two 

 or three oblong and blunt protuberances, and which 

 widens posteriorly into a clavate form abo\e the gill- 

 cover (on the OS squamosum), where it is interrupted 

 to lie again continued by an oblong protuberance, which 

 is smaller, but just as rough with small osseous warts, 

 on the posttempoi-al bone. This species also comes 

 nearest PI. Jlesus in the numl)er of rays in the dorsal 

 and anal fins (ZA 51 — 64; A. 37—45), though in this 

 respect, too, one of its varieties distinctly approximates 

 it to 1*1. plalessa. 



Thus we find here a group of two varieties which, 

 in its entirety as a species, forms a landmark between 



'■ Pai.l., Bussische Reise, Theil HI, Anliang, p. TOl!. 



'' Pall., Zoographla Bosso-Asiatica, toin. Ill, p. 292. 



■' Storer, Proe. Bost. Soc. Nat. Hist., I (1843), p. l.'iO. 



'' LiLLJ., Vet.-Akad. Handl. 1850, II. p. :!0G. 



' Gthr, Cat. Brit. Mus\, Fish., vol. IV. p. 442. 



■'■ Gill, Proc. Acad. Nat. Sc. Pliilad. 1864. p. 222. 



