650 
SCANDINAVIAN FISHES. 
most distinct in lean specimens, hardly perceptible in 
fat and fleshy ones. Thus in the former the lower jaw 
rises somewhat more rapidly in a forward direction, 
and projects beyond the true tip of the snout. Some- 
times too, especially in lean specimens, the said break 
renders the inferior profile of the head behind it almost 
horizontal. The length of the head is greater in the 
males than in the females, and varies individually be- 
tween 23 V 2 an< l 28 % of that of the body®. The eyes 
are round, and their superior margin lies almost exactly 
in the same plane as the interorbital space, which is 
flat or very slightly convex and in front narrows regu- 
larly, but only slightly; the line from the tip of the snout 
to the upper angle of the pectoral fin touches the lower 
margin of the pupil. The diameter of the eyes is equal 
to the breadth of the interorbital space* and in the fry, 
as usual, considerably greater than (sometimes 1 V 2 
times) the length of the snout, in old specimens equal to 
this length or at least 4 / 5 thereof. The simple, round 
nostrils, only one on each side, lie at the upper edges 
of the convex snout, about half as far from the eyes as 
from the tip of the snout. The length of the snout is 
in young specimens about 1 / 2 (or less), in old specimens 
(more than 1 / 2 dm. long) as much as ? ‘j sometimes 2 / 3 , 
of the postorbital length of the head, which length 
measures in young specimens about 3 / 5 (62 or 63 %), in 
old about 2 / 5 (40 — -48 %) of the total length of the head, 
or in the former (though not during the earliest stages) 
as much as 1 / s (50 — 46 %) in the latter only about 1 / 3 
(37 — 32 %) of the distance between the first spinous 
dorsal ray and the tip of the snout. The cleft of the 
mouth ascends obliquely and is rather small, the length 
of the upper jaw being equal to the diameter of the eyes 
or even less than it. Lips are not wanting; but they 
are not so tumid as in the Fifteen-spined Stickleback. 
Here too, the upper jaw is capable of protrusion by 
means of the fairly long nasal processes of the inter- 
maxillary bones, which processes extend, when drawn 
back, up to the groove in the upper surface of the snout, 
a little behind the line of union between the two nostrils. 
The length of the lower jaw is about equal to that of 
the snout, in young specimens considerably greater than 
the latter. The maxillary bones are narrow, only 
slightly widened at the hind extremity, and curved 
downwards, but about equal in length to the inter- 
maxillaries. When the mouth is closed, they are scarcely 
covered by the suborbital bones. Among the latter three 
bones the middle one is the smallest, and the hindmost, 
which is somewhat larger than the foremost (the pre- 
orbital bone), more or less completely fills the angle of 
the preoperculum, with its lower margin touching the 
horizontal branch of the last-mentioned bone, and some- 
times with the inferior part of its hind margin also 
touching the front of the vertical branch of the same, 
while a triangular process, sharply narrowing upwards, 
unites this hindmost suborbital bone to the posterior 
frontal bone. Thus, the preorbital bone, which forms 
the outer margin of the nasal cavity, being also united 
to the anterior frontal bone, the orbit is completely 
closed; and the naked patches on the cheeks, covered 
only by a glossy skin, have the same position as in the 
preceding species, though the anterior patch, behind the 
corner of the mouth, is much shorter. The jaw-teeth 
are of uniform size, set in front in several rows, behind 
in one or two, of uniform thickness, cylindrical, more 
or less blunt at the tip, and straight or only slightly 
curved. The pharyngeal teeth are of similar form, but 
somewhat larger and in general more pointed. The gill- 
rakers are pectinated, largest in the outer row on the 
front of the first branchial arch, where they are about 16 
in number, in the other rows small and scattered. The 
palatal folds within the jaws are well-developed both 
above and below. The tongue is more developed than 
in the preceding species, moderately fleshy, and broad 
(blunt at the tip). The structure of the opercular appa- 
ratus is the same as in the Fifteen-spined Stickleback, 
only that the anterior, horizontal branch of the preoper- 
culum and the interoperculum are shorter, and that the 
ascending process of the suboperculum between the oper- 
culum and the vertical branch of the preoperculum is 
larger. In form the operculum resembles a semicircle 
set on end and superiorly truncated at the end of the 
first third of its length or even a little earlier. The 
gill-openings are smaller than those of either the pre- 
ceding species or the following one, the branchiostegal 
membranes coalescing underneath with the isthmus 
throughout its length, back to the anterior extremity of 
the interclavicles, and thus being separated from each 
other. In the typical Three-spined Stickleback (the 
trachurus form) all the external bones of the head, as 
“ A comparison between 4 males and 8 females gives the result that in the males the length of the head is more, 
less, than 27 % of the length of the body. 
h In young specimens and in the gymnurus form the breadth of the forehead is comparatively less. 
in the females 
