534 SHARKS AND RAYS. 



and the anal tin is invariably wanting. In the more specialised forms the body is 

 greatly developed, and the pectoral fins 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 very 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) t 

 ' 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, 

 Other Genera. f . J; • • i 



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 (Laimargus 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 



