SHARKS AND RAYS. 515 



long conical clasper. The tail-fin is of the heterocercal type, with its upper 

 lobe which is traversed by the extremity of the back-bone greatly de- 

 veloped at the expense of the lower one. As all are aware who have ever 

 seen a shark turn over on its back to seize its prey, in most members of the 

 class the mouth is situated on the lower aspect of the head, some considerable 

 distance behind the tip of the muzzle. And a cruel mouth it is in the case 

 of ordinary sharks, where it is lined with row after row of sharp triangular 

 teeth, of which the outermost stand upright in readiness for immediate use, 

 whereas those of the innermost rows are recumbent, and not destined to see 

 active service until those near the margins have been worn out and shed. Some 

 sharks, like the Port Jackson species, have, however, the mouth placed in 

 the ordinary position at the extremity of the muzzle, while the teeth except 

 a few at the front of the jaws have low flattened, crowns, and form a mill- 

 like pavement adapted to grinding the shells of molluscs and crabs. Rays, 

 too, have pavement-like teeth of a still more markedly crushing type, 

 although in the males of certain kinds the individual denticules are 

 cusped. 



Sharks include the most predaceous and most dreaded of all fishes, and 

 many of the rays are likewise formidable monsters, which have the power of 

 inflicting terrible wounds by means of the poisonous spines arming the whip- 

 like tail. Bodily size is, however, by no means a criterion of offensive power 

 among the members of the sub-class, since the largest of all sharks, namely, 

 the basking-sharks, are harmless species, whose terminal mouths are armed 

 only with feeble teeth, and whose food consists chiefly of various invertebrate 

 animals. Skates and rays, which are bottom-haunting creatures, likewise 

 feed on invertebrates, their grinding teeth being specially adapted for crush- 

 ing shells. Although many kinds ascend tidal rivers for considerable dis- 

 tances, while a few have become adapted probably owing to physical 

 alterations on the earth's surface to a fresh-water mode of life, the Elasmo- 

 branchii in general are marine fishes. Although, as already stated, the rays 

 are found on the sea-bottom, while some sharks are met with at great depths, 

 the majority of the latter group are essentially pelagic creatures, pursuing 

 their prey at or near the surface of the open sea. The abundance of food to 

 be met with in the neighbourhood of frequented harbours renders these fishes 

 generally more numerous in such localities than elsewhere. 



ORDER I. SELACHIL 



SUB-ORDER I. ASTEROSPONDYLI. 



As all the existing members of the sub-class are placed in a single 

 ordinal group, it will be unnecessary to enter into the consideration of the 

 features by which that order is distinguished from the extinct groups now 

 included among the Elasmobranchii. It will accordingly suffice to point out 

 the distinctive features of the sub-order Asterospondyli, which includes the 

 greater number of the sharks. The group takes its name from the structure 

 of the bodies of the vertebrae, which in section show a star-like arrangement 

 of the calcareous plates forming their internal support, such radiating plates 

 being considerably more numerous than the circular ones running parallel to 

 the outer surface. The members of this sub-order are characterised exter- 



