PICKED DOG-FISH. 
1159 
(according to Kroyer 16 — 17 %) of the length of the 
head to 'the first gill-opening. Like the Tope, the 
Picked Dog-fish has an almost terete body, but the 
anterior part of the trunk is of the so-called hog-backed 
form, with a rounded triangular section. Behind the 
body is slightly compressed, the breadth being about 
equal to the depth, and the section almost quadran- 
gular, though rounded (convex) above and laterally 
sharpened below by a dermal carina, running on each 
side behind the perpendicular from the second dorsal 
fin and some way out above the beginning of the in- 
ferior caudal lobe. The sides of the tail are, hoAvever, 
without true median carina, nor are there any caudal 
grooves, though a trace thereof may sometimes be 
found above, at the dorsal margin, in front of the 
caudal fin. The head, in particular the translucent 
snout, is depressed, behind somewhat broader than the 
trunk, with breadth about equal to the depth of the 
latter, and with parabolically pointed snout. 
The length of the head to the first branchial aper- 
ture varies between 17 (in the nearly viable foetus 18) 
and 15 % of the length of the body. The length of the 
snout to the eyes measures about 2 / 5 (40 — 44 %) of the 
length of the head. Both the eyes and the orbits are 
oblong, the latter most so, the vertical diameter of the 
eyes being always more than 1 / 2 (53 — 70 %), that of 
the orbits sometimes 1 / 3 (41 — 33 % a ), of the longitudi- 
nal diameter. This measures in the case of the latter, 
which are besides sharply pointed at each end, about 
30' — 20 % of the length of the head. The pupil is 
either round, but acute-angled below, or transversely 
oblong. Behind the eyes, but higher (further in) than 
these, lie the rather large spiracles, their diameter being 
only slightly less than or even equal to the vertical 
diameter of the orbits, from which they are separated 
by a distance of the same magnitude. Their anterior 
margin is folded double, the fold being sometimes so 
deep as to give them the appearance of being really 
double. The interorbital width is generally about equal 
to the length of the snout, somewhat greater or less, 
usually the latter. The prone nostrils are set about 
half-way along the snout, but the distance from the 
mouth to the transverse line drawn through their pos- 
terior margins is greater in the young than that from 
the tip of the snout to the transverse line drawn through 
“ According to Kroyer down to 28 %. 
6 In a nearly viable foetus rather more than V 3 . 
their anterior margins. In the old the said distances 
are about equal, the tip of the snout in front of the 
nostrils being even relatively prolongated with age. 
The nostrils are, as usual, obliquely transversal, with 
a small pointed valvule at the middle of the anterior 
margin. Their width is about 1 / 3 of the distance be- 
tween them. The mouth is less curved than in most 
other Sharks, with deep folds around the corners. The 
distance from the tip of the snout to the anterior mar- 
gin of the mouth is about x / 2 , to the corners thereof 
about 60 — 70 of the length of the head. The teeth 
are densely set, without gaps or unpaired median tooth. 
They are merely attached to the skin, as in all the 
Plagiostoms, not to the cartilage, and are consequently 
mobile. The first two rows of teeth stand upright, 
apart from the rest, the inner row leaning backwards; 
the remaining 3 or 4 rows have the cusps turned inwards 
(backwards), covered by the transverse fold (velum) in the 
Fig. 338. One of the right teeth of the upper jaw ( a ) and the lower 
jaw (b) in the Picked Dog-fish ( Squalus acanthias), anterior aspect. X 5. 
front part of each jaw. The last-mentioned rows are, 
however, erected in the old, where the anterior rows 
have disappeared. Each tooth (fig. 338) is small — in 
an adult fish only about 2 mm. high — flat and sharp- 
edged, with a deep jag in the outer (posterior) margin, 
under the keen, laterally directed cusp. They have the 
same form in both jaws, but are somewhat larger in 
the lower than in the upper. In front they have a 
basal cusp running into the gum, and reminding us of 
the median ridge and the shaft with which the der- 
mal scales are furnished. In a young female Sundevall 
counted 26 teeth in each transverse row above and 
22 below. The tongue is not free, and consists mere- 
ly of a skin investing the rather large hyoid carti- 
lage, which occupies almost the whole mandibular arch. 
The branchial apertures are comparatively small, the 
middle ones smallest, the hindmost opening somewhat 
larger than the first, the height of which is about Vs 
to 7 2 greater than the vertical diameter of the orbits. 
Scandinavian Fishes. 
146 
