ARCTIC FLOUNDERS. 
405 
norii — with the surface of the body almost entirely 
smooth, with the exception of the weak spinous warts 
at the spots mentioned ancl 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 both species. 
We are apparently guided to this original type by a 
comparison between these two species and a third, 
Pleuronectes glacialis, which has a wide geographical 
range in the North, and is as variable as either of the 
others. This northern species has also borne many 
names: Pleuronectes glacialis a , PL cicatricosus b , Platessa 
glabra c , Platessa clvinensis' 1 , Pleuronectes frankliniP and 
Euchalarodus putncimi 1 . The first two names refer to 
the two distinct varieties (figs. 110 and 111) which 
Pallas knew from the European and Siberian coasts 
of the Arctic Ocean; the third, the fourth, and the last 
names were conferred on the species on the Atlantic 
coast of North America — where it has been found as 
far south as Salem — and on the Arctic coast of Eu- 
rope. The last name but one has 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 been found. 
Pleuronectes glacialis is distinguished both from the 
Plaice and the Flounder by the greater depth of the tail, 
the least depth of which is always more than 9 % of the 
length of the body — a measurement we have found in 
the Plaice only exceptionally, and never quite exactly, 
though very nearly so. This greater depth of the tail 
gives to PL glacialis the same character as PL limanda 
acquires by 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 PL platessa and PL 
f/esus this proportion is always — to the best of our 
knowledge with only one single exception — less than 
35 %. Another character, which on the other hand 
really ranges PL glacialis in immediate systematic prox- 
imity to PL limanda, consists in the scales of the body. 
By far the greater part of the eye side, or at least 
those parts where the scaly covering is strongest in the 
a Pall., Russische Rei.se, Theil III, Anhatig, p. 706. 
'' Pall., Zoograplria Rosso- Asiatica, tom. Ill, p. 292. 
' Storer, Proe. Dost. Soc. Nat. Hist., I (1843), p. 130. 
d Lillj., Vet.-Akad. Hand]. 1850, II. p. 306. 
e Gthr, Cat. Brit. Mus., Fish., vol. IV, p. 442. 
J Gill, Proc. Acad. Nat. Sc. Philad. 1864, p. 222. 
two preceding species, are clothed with ciliated scales, 
in type the same as those of PL limanda, but shorter 
(smaller) and scattered, never imbricated, but rather set 
in a quincunx; and as a rule, one row of these scales 
advances at least over the middle rays of the dorsal 
and anal fins. As usual, however, the scales may also 
vary in PL glacialis; and sometimes the ciliated scales 
occur only on the eye side, in a row along the bases 
of the dorsal and anal fins, while they are sometimes 
close-set and contiguous, but not imbricated, 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 vertebrae, too, PL glacialis comes nearest PL limanda. 
According to our observations the former possesses 38 
or 39 (according to Richardson 40) vertebrae, while 
PL limanda has 39 or 40. In this respect, therefore, 
both PL glacialis and PL limanda appear as inter- 
mediate forms between PL Jiesus , with 35 — 37 vertebrae, 
and PL platessa , with 43 or 44. By the shape of the 
lower pharyngeals and the structure of their teeth, how- 
ever, PL glacialis is ranged in the same group as PL 
Jiesus and PL platessa; and in this respect it comes 
nearest the Plaice, in which these bones are narrower 
than in the Flounder. PL glacialis also comes nearest 
PL platessa in the number of gill-rakers, which varies 
on the first branchial arch between 9 and 12. In the 
cephalic characters PL glacialis 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 elavate form above the gill- 
cover (on the os squamosum), where it is interrupted 
to be again continued by an oblong protuberance, which 
is smaller, but just as rough with small osseous wart 3, 
on the posttemporal bone. This species also comes 
nearest PL Jiesus in the number of rays in the dorsal 
and anal fins (D. 51 — 64; A. 37 — 45), though in this 
respect, too, one of its varieties distinctly approximates 
it to PL platessa. 
Thus we find here a group of two varieties which, 
in its entirety as a species, forms a landmark between 
