fi50 



SCANDINAVIAN FISIIKS. 



most distinct in lean specimens, hardly perceptible in 

 fat and fleshy ones. Thus in the former the lower jaw 

 i-ises somewhat more I'apidly 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. Tiie len>:th of tiie head is greater in the 

 males than in the females, and %'aries individually be- 

 tween 23' 2 and 28 % of that of the body". The eyes 

 are round, and their supei'ior 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 ey(>s is equal 

 to the breadth of the interorbital space* and in the fry, 

 as usual, considerably greater than (sometimes 1' ., 

 times) the length of the snout, in old specimens equal to 

 this length or at least ^/s 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 e3'es as 

 from the tip of the snout. The length of the snout is 

 in young specimens about \ ^ (or less), in old specimens 

 (move than '/^ dm. long) as much as 7.i> sometimes "V^, 

 of the postorbital length of the head, which lengtli 

 measures in young specimens about Vs (62 or 63 %), in 

 old about - - (40 — 48 %) of the total length of the head, 

 or in the former (though not during the earliest stages) 

 as much as ' \. (.50 — 46 %) in the latter only about '/g 

 (37 — 32 %) of the distance Ijetween 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 l)y 

 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 greatei' than 

 the latter. The raaxillar)- bones are narrow, onlj' 

 slightly widened at the hind extremity, and curved 

 downwards, but about equal in length to the inter- 



maxillaries. When tiie mouth is closed, thev are scarcely 

 covered by the suborbital bones. Among the latter three 

 liones the middle one is the smallest, and the hindmost, 

 wliich is somewhat larger than the foremost (the pre- 

 orbital bone), more or less com]detely fills the angle of 

 the preojiei'culum, with its lower margin touching the 

 horizontal l)ranch 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, whicli forms 

 the outer margin of the nasal cavity, being also united 

 to the anterior frontal bone, the orbit is completely 

 closed; and the naked ])atches on the cheeks, covered 

 only by a glossy skin, have the same ])Osition as in the 

 preceding species, though tlie anterior jjatch, behind tiie 

 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 unifoi-m thickness, cvlindrical, more 

 or less blunt at the tip, and straight or only slightly 

 curved. The pharyngeal teeth are of similar form, l)ut 

 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, moderatelj- 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 liranch of the preoper- 

 culum and the interoperculum are shorter, and that the 

 ascending process of the suboperculuin between the o])er- 

 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. Tiie 

 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 extrerait)- of 

 the iuterclavicles, and thus being separated from each 

 other. In the typical Three-spin ed Stickleback (the 

 trachunis form) all the external bones of the head, as 



" A conipaiison between 4 males nml 8 females gives the result tliat in the males the lengtli of the head is more, in the females 

 less, than 27 % of the length of the body. 



'' In young specimens and in tlie iji/inuuntft form the breadth of the forehead is comparatively less. 



