Encyclopaedia Britannica > Volume 21, ROT-Siam

3 and 5). They are all oviparous, their oblong egg-shells
being produced at each corner into a long thread by which the
egg is fastened to some fixed object. Some of the tropical
species are ornamented with
a pretty pattern of coloration.
The two British species, the
Lesser and the LargerSpotted
Dog-Fish (Sc. canicula and
Sc. catulus), belong to the
most common fishes of the
coast, and are often con¬
founded with each other.
But the former is finely dotted
with brown above, the latter having the same parts covered
with larger rounded brown spots, some of which are nearly
775
Fig. 3.—Teeth of Scyllium canicula.
Fig. 4.—Chiloscyllium trispeculare.
as large as the eye. As regards size, the latter exceeds
somewhat the other species, attaining to a length of 4 feet
Dogfishes may become
extremely troublesome
by the large numbers in
which they congregate at
fishing stations; nor do
they compensate for the
injury they cause to
fishermen, being but
rarely used as food, ex¬
cept at certain seasons
by the poorer classes
of the Mediterranean
countries, in China and
Japan, and in the Ork-
neys, where they are dried Fig- 5.—Confluent Nasal and Buccal Cavities
for home consumption. of the same ^
The Black-mouthed Dog-Fish (Pristiurus melanostomus) is
another European species which is rarely caught on the
British coasts, and is recognized by a series of small, flat
spines with which each side of the upper edge of the caudal
fin is armed.
The Tiger-Shark (Stegostoma tigrinum) is one of the
commonest and handsomest sharks in the Indian Ocean.
The ground colour is a brownish-yellow, and the whole fish
is ornamented with black or brown transverse bands or
rounded spots. It is a littoral species, but adult specimens,
which are from 10 to 15 feet long, are not rarely met far
from land. It is easily recognized by its enormously long
bladelike tail, which is half as long as the whole fish. The
teeth are small, trilobed, in many series. The fourth and
fifth gill-openings are close together.
The genus Crossorhinus, of which three species are known
from the coasts of Australia and Japan, is remarkable as
the only instance in this group of fishes in which the in¬
teguments give these inactive ground-sharks, whilst they lie
concealed watching for their prey, what may be called a
“ celative ” rather than a “ protective” resemblance to their
surroundings. Skinny frond-like appendages are developed
near the angle of the mouth, or form a wreath round the
side of the head, and the irregular and varied coloration of
the whole body closely assimilates that of a rock covered
with short vegetable and coralline growth. This peculiar
development reminds us of the similar condition in the sea-
devil (Lophius), where it serves also to conceal the fish from
its prey, rather than to protect it from its enemies. The
species of Crossorhinus grow to a length of 10 feet.
The so-called Port Jackson Shark (Cestracion) is likewise
a littoral form. Besides the common species (C. philippi),
Fig. Cestracion galeatus.
three other closely allied kinds from the Indo-Pacific are
nown. This genus, which is the only existing type of a
separate family, is one of special interest, as similar forms
occur in Primary and Secondary strata. The jaws are
armed with small obtuse teeth in front, which in young
individuals are pointed, and provided with from three to
five cusps The lateral teeth are larger, pad-like, twice as
broad as long and arranged in oblique series (fig. 7),
an
Fig. 7.—Upper Jaw of Port Jackson Shark (Cestracionphilippi). (x 5.)
arrangement admirably adapted for the prehension and
mastication of crustaceans and hard-shelled animals. The
fossil forms far exceeded in size the living, which scarcely
attain to a length of 5 feet. The shells of their eggs are
not rare in collections, being found thrown ashore like those
of our dog-fishes. The shell is pyriform, with two broad
lamellar ridges each wound edgewise five times round it
(fig. 8).
The Spiny or Piked Dog-Fish (Acanthias) inhabits, like
the majority of littoral genera of sharks, the temperate
seas of both the northern and southern hemispheres. For
some part of the year it lives in deeper water than the
sharks already noticed, but at uncertain irregular times it
appears at the surface and close inshore in almost incredible
numbers. Couch says that he has heard of 20,000 having
been taken in a sean at one time ; and in March 1858 the
newspapers reported a prodigious shoal reaching westward
to Uig, whence it extended from 20 to 30 miles seaward,
and in an unbroken phalanx eastward to Moray, Banff, and
Aberdeen. In the deep fjords of Norway, and indeed
at every station of which a shoal of these fishes has taken
temporary possession, line-fishing has to be suspended
during the time of their visit, as they cut the lines with
their scissors-like teeth. As expressed by the name, these