42 
SCANDINAVIAN FISHES. 
forehead, one on each side of either nostril. One row 
of cavities runs along the cheek, and another on the 
margin of the preoperculum is continued along the lower 
jaw. In front of all, on each side of the tip of the 
snout, is a little, round hole through which the system 
of ducts has free communication with the surrounding 
water. The operculum and preoperculum, as well as 
the shoulder-girdle (the post-temporal and clavicular 
bones) are armed with spines. The mouth is of average 
size, and the upper jaw projects slightly over the lower. 
There are several rows of very fine and closely set 
teeth on the jaws and the anterior portion of the palate, 
and also 4 pharyngeal patches of teeth, 2 upper and 2 
lower. The nasal sacs are fairly elongated and are 
situated about halfway between the tip of the snout 
and the anterior orbital margin. The anterior nostril 
is tubular arid round and is furnished with a mem- 
branous flap, the posterior is larger and oval and is 
without a cover. The eyes are fairly large, their lon- 
gitudinal diameter is in older specimens 24 or 25 % of 
the length of the head, in younger about 29 %. In the 
former this diameter is equal to the distance between 
both the posterior nostrils, in the latter greater than it. 
The iris is yellow, the lens blue. When the fish has 
been out of the water for some time, however, the lens 
turns a whitish blue-green, something like the appea- 
rance known as wall-eye. The lateral line is near the 
back and follows its curve. In the anterior portion of 
the body it pierces the scales obliquely downwards in 
a longitudinal direction, so that the anterior scales in 
this line are dentated at the posterior margin only above 
the ducts of the lateral line, and are also posteriorly 
emarginate. Nearer the tail the ducts of the lateral 
line pierce the scales nearer their centre and follow 
their direction exactly, so that the hind margin is den- 
tated both above and below the incision we have just 
mentioned. The belly is yellowish white and flat, as is 
also the breast, which is silver -white but shifts in colour 
like mother of pearl at the death of the fish and finally 
becomes rose-red. In the dorsal fin there are from 25 
to 27 rays, and of these the posterior ones, from 10 
to 13 in number, are soft and branched, while the rest 
are strong and pungent spinous rays. In the pectoral 
fins there are, as a rule, 14 or 15 rays, and of these 
the two highest, and generally the two lowest as well, 
are undivided but articulated. In colour these fins are 
grayish yellow with fine, dark dots forming transverse 
bands. The ventral fins are almost white and without 
spots. The anal fin resembles the ventral fins in colour 
or sometimes has just a few dark spots upon it. As 
a rule, it contains only 2 spinous rays, and, a strange 
exception to the general rule, the first of these is con- 
siderably longer and stronger than the other. In the 
specimen which has 3 spinous rays in this fin, the first 
is little more than half as long as the second, which 
is the strongest and longest. The caudal fin, which is 
grayish yellow with dark spots arranged in transverse 
bands, is deeply forked. 
The internal organs of this fish are very like those 
of the Perch, with the exception of the fact that in 
the female the roe-sac is double, though its parts are 
united posteriorly. The number of the vertebrae is 37 
and of the ribs 15. 
We know just as little of any constant, external 
distinction between the sexes in the Pope as in the Perch 
and Pike-perch, but difference of age involves conside- 
rable changes in form, and some of the most important 
of these may be traced in the following table: . 
Length of the body from the tip of the snout to the end of the middle rnvs of the caudal tin mm. 
Length of the head in % of the length of the body 
Longitudinal diameter of the eye » » » » » » s » 
Base of the dorsal fin »»»» s> » » » 
Spinous-rayed portion of the dorsal fin _ » » » s » j> » » 
Base of the anal fin »»»» » » 
Distance between the insertion of the ventral fins and the beginning of the anal fin » » » » » » * » 
Length of the ventral fins d » » .» s * » » 
Greatest depth of body » » ;> » » » » » 
Least j> » » » » » * » » » » 
Acerina cernua from 
Siljan 
9 
-a 'S 
JXj 
Islands 
near 
Stock- 
holm a 
9 
Mbrko 
9 
83 
137 
142 
167 
28,9 
27 
28,9 
27,5 
8,4 
6,6 
■7,4 
6,6 
45,8 
48,2 
50,7 
51,5 
28,9 
31,4 
35,2 
33,5 
10 
11,7 
13,4 
12,3 
28,9 
31,4 
31,3 
32,3 
16,9 
19 
17,6 
18,5 
21,7 
27 
27,4 
27 
7 
8 
8,4 
7,8 
16 3 
In this specimen we find D. —and A.—, as well as a number of juvenile characters, e. g. the exceptional size of the head and of the eyes. 
