892 
SCANDINAVIAN FISHES. 
rakers are scattered and short, but that, where the diet 
is composed of small objects, principally Entomostraca, 
they are finer, denser, and longer, thus forming a more 
perfect filtering-apparatus. Now as the diet is com- 
monly altered with the age of the Gwyniads, we may 
as a rule expect to find a corresponding change in the 
gill-rakers". But Gwyniad forms occur, for example 
the Siberian pelet (among the Vendaces) and mulcsun 
(among the true Gwyniads) or the so-called asp of Lap- 
land, which attain a considerable size, but have per- 
sistently dense and numerous gill-rakers. Some forms, 
again, show an increase with age in the number of the 
gill-rakers 6 . The variations do not therefore follow in 
absolute succession the changes of growth. They rather 
shows a certain independence in their appearance, an 
independence 'which in some forms has rendered them 
available as characters, at least in their extremes, espe- 
cially as they are accompanied in most cases by an 
external character, a difference in the shape of the snout. 
By the last-mentioned character two groups have 
long been distinguished within the genus Coregonus, 
one of them having its best-known representative in 
our Vendace, the other in our Gwyniad. To the for- 
mer group Agassiz 0 gave the name of Argyrosomus, 
characterized essentially by the projection of the point 
of the lower jaw beyond the tip of the snout, whereas 
the form-series of the true Gwyniads advances to a 
greater development and protrusion of the tip of the 
snout, culminating in the form hence known as the 
ndbbsik ( Beaked Gwyniad , Coregonus oxyrhynclnis). The 
limit between the two groups it is indeed impossible 
sharply to define, on account of the transition forms; 
but in its two extremes the altered shape of the snout 
is due to a very considerable difference in the form 
and position of the intermaxi llaries. We have already 
remarked in the case of Thymallus a considerable re- 
duction of the intermaxillaries, to small, flat and thin, 
triangular disks, set transversely in front of the tip of 
the snout and the incurved articular processes of the 
maxillaries, slightly thickened at the lower margin 
alone, where lie the alveoli of the single row of teeth, 
which are directed inwards (backwards), more or less 
vertically to the plane of the disk. In the Vendaces 
this structure recurs, only that the teeth are as a 
rule wanting, the disk is still thinner and rests more 
entirely on the front of the inward, terete tip of the 
maxillary bone. This part of the maxillary bone some- 
times, as in the Scandinavian Vendace, bears outside 
the protuberance which articulates with the ethmoidal 
cartilage, a separate process, directed downwards (for- 
wards), with which the inner (hind) surface of the inter- 
maxillary bone articulates, and by means of which the 
said bone is raised when the mouth is opened. The 
breadth (height) of the intermaxillary bone does not 
exceed 2 /s °f the breadth of the snout across the ar- 
ticular knobs of the maxillaries, and its sharp inferior 
margin then forms the osseous framework in the sharp, 
transverse margin of the tip of the snout. In the true 
Gwyniads the intermaxillary bone on each side of the 
snout is higher, the depth of the snout being always 
more than 2 / 5 of its breadth across the articular knobs 
of the maxillaries, and more robust, with teeth or ru- 
diments thereof in the above-mentioned position. It is 
also more firmly articulated with the under surface of 
the tip (articular process) of the maxillary bone. This 
articulation is formed in the following manner: the 
upper margin of the intermaxillary bone is hollowed 
into a groove which surrounds the said lower margin 
of the maxillary not only in front, but also more or 
less far back (outwards) on that part of the latter bone 
which forms the upper lateral margin of the mouth. 
The intermaxillary bone thus assumes a more or less 
vertical position when the mouth is closed; and when 
the tip of the snout is prolongated — which often ap- 
pears as a change of growth — the under margin of this 
bone is turned more and more in a backward direction. 
The scales of Coregonus are thin and cycloid, as 
a. rule thinner than those of the Grayling, with not very 
distinct radiating grooves, though both the anterior and 
posterior margins of the scales are generally corrugated 
and notched thereby. The scales of the lateral line, 
here as in the Grayling, commonly have the anterior 
margin elongated to a triangle at the middle. 
The internal organs resemble those of the Salmons; 
but the pyloric appendages are shorter and still more 
numerous (Kroner counted about 200). The perito- 
neum is silvery white. 
The genus has the same geographical extension as 
the Graylings. It is most numerous, and appears in 
its most developed forms, in the Siberian rivers, which 
a See for example the four Gwyniads, 206 — 350 mm. long, which. are included in Smitt, Riksm. Salmon., tab. melr. X, Nos. 170 — 173. 
b See for example Smitt, 1. c., p. 278, the averages for Coregonus Nilssonii and Cor. Wartmanni. 
c Lake Superior, p. 339. 
