Encyclopaedia Britannica > Volume 21, ROT-Siam

778
SHARK
same family as the spiked dog-fish, but grows to a much
larger size, specimens 15 feet long being frequently met
Fig. 15.—Dentition of Greenland Shark.
Fig. 14.—Greenland Shark {Lxmargus borealis).
with. The two dorsal fins are small and destitute of
spines. The teeth (fig. 11) in the upper jaw are small,
narrow, conical in shape ; those of the lower flat, arranged
in several series, one on the top of the other, so that only
the uppermost forms the sharp dental edge of the jaw.
The points of
these lower
teeth are so
much turned
aside that the
inner margin
only enters the
dental edge.
The Green¬
land shark is
an inhabitant
of the Arctic
regions, some¬
times straying
to the lati¬
tudes of Great
Britain and of
Cape Cod in
the western Atlantic ; it is one of the greatest enemies of
the whale, which is often found with large pieces bitten
out of the tail by this shark. Its voracity is so great that,
as Scoresby tells us, it is absolutely fearless in the presence
of man whilst engaged in feeding on the carcase of a
whale, and that it will allow itself to be stabbed with a
lance or knife without being driven away.
The Spinous Shark (Echinorhinus spinosus) is readily re¬
cognized by the short bulky form of its body, its short tail,
and the large round bony tubercles which are scattered all
over its body, each of which is raised in the middle into a
pointed conical spine. More frequent in the Mediterranean,
it has been found also not very rarely on the English coasts
and near the Cape of Good Hope. It is always living on
the ground, and probably descends to some depth. It does
not seem to exceed a length of 10 feet.
Bathybial Sharks.—Sharks do not appear to have yet
reached the greatest depths of the ocean ; and so far as we
know at present we have to fix the limit of their vertical
distribution at 500 fathoms. Those which we find to have
reached or to pass the 100 fathoms line belong to generic
types which, if they include littoral species, are ground-
sharks,—as we generally find the bottom-feeders of our
littoral fauna much more strongly represented in the deep
sea than the surface swimmers. All belong to two families
only, the Scylliidee, and Spinacidae, the littoral members of
which live for the greater part habitually on the bottom
and probably frequently reach to the 100 fathoms line.
Distinctly bathybial species are two small dog-fishes,—
Spinax granulatus from 120 fathoms, and Scyllium
canescens from 400 fathoms, both on the south-west coast
of South America ; also Centroscyllium granulatum from
245 fathoms in the Antarctic Ocean, whose congener from
the coast of Greenland probably descends to a similar
depth. The sharks which reach the greatest depth
recorded hitherto belong to the genus Centrophorus, of
which some ten species are known, all from deep water in
the North Atlantic, Mediterranean, the Molucca and
Japanese seas. The Japanese species were discovered by
the naturalists of the “Challenger” on the Hyalonema
ground off Inosima in 345 fathoms. Dr E. P. Wright
found G. coelolepis at a still greater depth on the coast of
Portugal. The fishermen of Setubal fish for these sharks
in 400 or 500 fathoms, with a line of some 600 fathoms
in length. “ The sharks caught were from 3 to 4 feet long,
and when they were hauled into the boat fell down into
it like so many dead pigs”; in fact, on being rapidly
withdrawn from the great pressure under which they
lived they were killed, like other deep-sea fishes under
similar circumstances. It is noteworthy that the organiz¬
ation of none of these deep-sea sharks has undergone
such a modification as would lead us to infer that they
arc inhabitants of
great depths.
One of the
most interesting
types of the divi¬
sion of sharks is
the small family
of Notidanidx,
which is external¬
ly distinguished
by the presence
of a single dorsal
fin only, without
spine and oppo¬
site to the anal,
and by having
six or seven wide
branchial open¬
ings. They repre¬
sent an ancient
type, the presence
of which in Ju¬
rassic formations
is shown by teeth
extremely similar
to those of the
living species.
Their skeleton
is notochordal.
Only four species
are known, of
which one (JYoti-
danus griseus) has
now and then
strayed north¬
wards to the
Fig. XG.—Chlamydoselachus anguineus.
English coast. A member of this family has been re¬
cently discovered in Japan, and is so scarce that only
two specimens are known—one in the museum at Cam¬
bridge, U.S., and the other in the British Museum. It
was named by its first describer, S. Garman, Chlamy¬
doselachus anguineus (fig. 16). It resembles somewhat
in shape a conger, and differs from the Notidani proper
by its elongate body, wide lateral and terminal mouth,
extremely wide gill-openings, and peculiarly formed teeth.
The teeth are similar in both jaws, each composed of
three slender curved cusps separated by a pair of rudi¬
mentary points, and with a broad base directed back¬
wards. These teeth resemble some fossils of the Middle
Devonian, described as Cladodus, and North-American