534 
SHARKS AND RAYS. 
Spiny Dog-Fish. 
and the anal tin is invariably wanting. In the more specialised forms the body is 
greatly developed, and the pectoral tins attain an enormous development; while the 
spiracles are of large size, and always retained. The present family includes the 
most generalised members of the group, in which the body is cylindrical or 
triangular, and but verj^ slightly depressed; the mouth being gently arched, and 
the muzzle blunt. The pectoral fins have no forward prolongation, and are not 
notched at their point of origin; and the small and lateral gill-clefts may be 
either in the line of the pectorals, or half below. The large spiracles are placed 
behind the eyes; there is no nictitating membrane to the eye; and the two dorsal 
fins may or may not be provided with spines. 
The common spiny, or picked, dog-fish (Acanthias vulgaris ) a 
shown in the upper figure of the illustration on p. 525, is the most 
familiar representative of a very small genus characterised by the presence of 
spines to the dorsal fins, and by the peculiar form of the teeth, which are similar 
in the two jaws, and small, triangular, and compressed, with the points much turned 
aside, and the cutting-edge formed by the inner margin. The common species 
measures from 3 to 4 feet in length, and is slaty blue above, and yellowish white 
beneath. It is very abundant on the British coasts, sometimes making its appear¬ 
ance in such incredible numbers that upwards of twenty thousand were once 
captured in a single haul on the Cornish coast. In common with an allied species 
(A. blainvillei), this dog-fish presents the peculiarity of inhabiting the two 
temperate zones but being unknown in the intervening tropical seas. The eggs 
are hatched within the body of the female, and a considerable number of young 
are produced at a birth. Somewhat dangerous wounds result from the spines. 
Among other types, we may notice the genus Centrophorus, 
represented by eight European species, and a ninth from the Moluccas, 
all of which differ from the last by the upper teeth being erect and spear-like, with 
a single cusp; the dorsal spines being often very small. Apparently not exceeding 
5 feet in length, these sharks are noteworthy on account of the depth at which 
they live; one of the species being caught with lines at a depth of from three to 
four hundred fathoms off' the coast of Portugal. When hauled up, these fish are 
quite dead, owing to the diminished pressure. A fossil species occurs in the Chalk 
of Syria. In the typical genus Spinax the teeth in the two jaws are likewise 
dissimilar; but those of the lower one are broader than in the last, although with 
the points similarly turned aside. The genus is now represented by three small 
species from the Atlantic and the extremity of South America; but has been 
recorded from the Miocene Tertiary. The Greenland shark (Lcemargus borealis ) 
of the Arctic seas, which occasionally strays as far south as Britain, represents 
another genus characterised by the small size of all the fins and the want of spines 
to the dorsals, the first of which is situated considerably in advance of the pelvic 
pair; the skin being uniformly covered with small tubercles. In the upper jaw 
the teeth are small, narrow, and conical; but those of the lower jaw, which are 
numerous and form several series, have their points so much bent to one side that 
their inner margins form the cutting-edge, which is not serrated. Growing to a 
length of 15 feet, the Greenland shark is a determined enemy to the right whale 
of the same seas; and when feeding on the carcase of one of those mammals 
Other Genera. 
