1140 
SCANDINAVIAN FISHES. 
a very small flap (valvule) at the anterior margin. 
They lie so far in front of the perpendicular from the 
preorbital margin that the distance between them and 
the tip of the snout is about 80 — 90 % of that between 
the mouth and the same point, which latter distance 
is rather less than or sometimes equal to the length 
of the snout. The mouth is large, its breadth at the 
corners being somewhat more than 1 j 3 of the length of 
the head. The cleft of the mouth is sharply curved, 
so that the lower jaw is rather long, and the distance 
from the corners of the mouth to the middle of the 
anterior margin of the upper jaw is about 3 / 10 of the 
length of the head. Externally no dermal folds are 
visible on the jaws, for the small creases that lie above 
and below each corner of the mouth are concealed by 
Fig. 330. Head of a Porbeagle (Isarus cornubicus) 213 cm. long, 
seen from below, ft nat. size. 
the skin of the upper jaw. The teeth (tig. 329) are 
set in three rows. The anterior ones are the largest, 
measuring in a specimen 24 dm. long somewhat more 
than 12 mm. or 1 / 3 of the diameter of the eyes. They 
are set rather far apart, of a narrow triangular form or 
conical, terete on the inner, but somewhat flattened on 
the outer side. The base of the broad root is emarginate, 
so that the tooth apparently has two fangs. In old 
specimens the root has a small conical cusp on each 
side; but this cusp is wanting in young specimens, 
sometimes even at a length of 9 dm. In both jaws 
we And at the extreme front a small gap,, without 
middle tooth. The upper jaw has 14 or 15 (1G) teeth 
in each row on either side, the number being alike 
both in young and old. The first two teeth are large, 
the third is rather small — and behind this lies a wide 
gap ( diastema ) — the fourth and the following ones are 
again large, but gradually decrease in size, the hind- 
most teeth being comparatively minute. In the lower 
jaw there are 12 or 13 (14) teeth on each side. The 
gap at the extreme front is somewhat wider than in 
the upper jaw. The first and second teeth are the 
largest, the others decreasing in size behind. All the 
teeth are slightly recurved. 
The spiracle of the Porbeagle is extremely small, 
resembling a hole pierced with a coarse pin. It lies 
exactly behind the eye, separated therefrom by a dis- 
tance of twice the diameter thereof or somewhat more 
in large specimens. It is consequently situated rather 
further back than in most of the other Sharks, and has 
many times defied detection, being probably obliterated. 
All five branchial apertures lie in front of the pectoral 
tin, the lower end of the hindmost opening terminating 
close below the margin of this fin. This opening is, 
however, obliquely set, being considerably approximated 
below to the penultimate aperture. In the dermal fold 
that covers each opening, lie three parallel cartilaginous 
rays (radiating extrabranchial cartilages, see above), 
originating from the middle of each arch and extend- 
ing to the edge of the opening. 
The body itself is almost terete, with indistinct 
lateral line, which is best marked in dried skins. From 
the temporal region, above and behind the eyes, the 
line forms a slight curve above the gills and then pro- 
ceeds in a straight direction to the lateral carinae of 
the tail. These occupy, immediately in front of the 
caudal tin, one-fifth of the length of the body behind 
the head. They have an almost sharp, curved outer 
margin, and are so large that their breadth is 2 / 3 of the 
greatest breadth of the body, arid that the tail just in 
front of the caudal grooves is more than four times as 
broad as deep. The caudal grooves above and below are 
deep and clothed with finer scales than the rest of 
the skin. 
The first, dorsal tin is rather large, its base mea- 
suring about V 9 , the length of its anterior margin 1 / 7 
— V 6 , and its height about •■/ 8 , of the entire length of 
the body. Its beginning lies at a distance from the 
tip of the snout measuring about 30 — 32 % of the length 
of the body. In form it resembles an almost equilateral 
triangle, with a large rounded incision above the elon- 
gated posterior angle. The posterior dorsal and the 
anal fins are almost similar in form and size, and lie 
