1058 
SCANDINAVIAN FISHES. 
dorsocaudal row. The plates of the ventral lines vary 
in each row within the preabdominal region, as a rule, 
between 10 and 12; but Krdyer once found only 8, and 
Liel.jeborg adduces as many as 14. Behind the vent 
there generally lie 2 or 3 pairs in front of the anal 
fulcrum, or the posterior of these pairs coalesces into 
one plate. Behind the anal tin are set 3 — 5 (in excep- 
tional cases only 2) pairs of distinct plates in front of 
the lower caudal fulcrum ; but to the posterior part of 
the anal fin the same remarks apply as to the corre- 
sponding part of the dorsal. In the preceding pages 
we have also considered the transformation of the re- 
maining dermal covering from small scattered spines 
and their more or less distinct agglomerations, simul- 
taneously with their adoption of a blunter, more tuber- 
culate form, into the diamond-shaped dermal scutes, 
arranged in oblique transverse rows, with small rows of 
tubercles between them, that are especially prominent 
on the dorsal sides above the lateral lines. On the tins 
and their rays the small, sharp spines are persistent; 
but on the upper part of the caudal tin, on each side 
of the continuation of the spinal column within this 
tin, elongated, diamond-shaped, and smooth bony plates 
are developed, arranged in oblique rows running from 
below and in front upwards and backwards. 
The hind part of the head is rather terete, con- 
stricted above at the gill-openings and eyes, with more 
or less depressed interorbital space. A striking diffe- 
rence from the head of the Teleosts is that the opercula 
do not entirely cover the gill-openings, the gills of the 
Sturgeon being consequently laid bare to some extent 
behind. In front the head is depressed and of varying 
length, tapering in a pyramidical form, with the superior 
surface of the snout more convex than the inferior and 1 
the other two sides narrowing sometimes to an edge. 
In young Sturgeons the relative length of the snout, as 
we have mentioned above, is considerably greater than 
in old, varying with age from about 14 % at least to 
about 8 3 / 4 % of the length of the body, or from 53 % 
(according to Kroyer sometimes 57 %) to 42 % of that 
of the head, so that the entire length of the head is also 
comparatively less in the latter. The postorbital length 
of the head, on the other hand, relatively remains al- 
most unaltered during growth, being about V ]0 a °f the 
length of the body. The eyes are small and somewhat 
oval. Their longitudinal diameter, which shows relative 
diminution during the growth of the fish, varies between 
10 and 7 % of the length of the head, between 19 and 
14 % of the length of the snout, or between 37 and 22 % 
of the interorbital width 6 . The interorbital space, which 
is convex, but medially more or less depressed, grows 
narrower in proportion to the general growth, varying 
between about 7 l / 2 and 6V 2 % of the length of the body, 
but wider in proportion to the length of the head, 
varying between about 27 and 33 % thereof.' In front 
of the eyes lie the large nostrils — comparatively larger, 
however, in young Sturgeons than in old — the posterior 
in each pair being the larger, an obliquely or even a 
transversely set ellipse, the length of which is some- 
times greater than the diameter of the eyes, and situated 
low down; the anterior more extended in the longitu- 
dinal direction of the body, and set nearer to the level 
of the forehead. They are separated by a thin (in old 
specimens rather broad) dermal ridge. The shortening 
of the snout during the growth of the fish is accom- 
plished at the expense of its anterior part (the rostral 
cartilage), the distance from the anterior nostril to the 
tip of the snout being sometimes reduced with age 
from 47 to 28 % of the length of the head. 
The under surface of the head, like that of the 
belly, is rather plane. Behind the perpendicular from 
the centre of the eyes is situated the comparatively 
small mouth 0 . Owing to its structure, which we have 
described above, it may be protruded (in a downward 
direction), so as to form a square tube, rounded, how- 
ever, at the corners (fig. 292). The toothless jaw-margins 
are rounded. The lips are tumid, on the upper jaw 
rather uniformly, but with a median indentation, at the 
corners of the mouth in a bulging form, on the lower 
jaw in the form of two fleshy folds, which are closely 
applied to each other from the sides, but distinct, the 
jaw itself being medially naked. Half-way between the 
tip of the snout and the mouth — sometimes, in young 
Sturgeons, distinctly nearer to the latter, sometimes, in 
old, nearer to the former d — hangs the transverse series 
of barbels characteristic of the Sturgeons. These are 4 
a Varying, according to our measurements, between about lO 1 /^ and 9 1 9 /. 
b For the vertical diameter of the eyes the corresponding percentages are respectively 8 and 5'/ 2 ! 16 and 12 , 30 and 16 . 
c One of the most reliable distinctions from the Hausen ( Acipenscr huso ) and the Osse'tr ( Tok , Ac. Gulden stcidtii), which have a 
broader mouth. 
d The distance between the barbels and the tip of the snout measures 64 — 44 % of that between the mouth and the same point. 
