336 FISHES. 



to pursue and catch rapidly-moving animals ; therefore they 

 feed chiefly on molluscous and crustaceous animals. How- 

 ever, the colour of their integuments assimilates so closely 

 that of their surroundings, that other fishes approach them 

 near enough to be captured by them. The mouth of Eays 

 being entirely at the lower surface of the head, the prey is 

 not directly seized with the jaws ; but the fish darts over its 

 victim so as to cover and hold it down with its body, when it 

 is conveyed by some rapid motions to the mouth. 



Eays do not descend to the same depth as Sharks ; with one 

 exception,^ at least, none have been known to have been caught 

 by a dredge worked in more than 100 fathoms. The majority 

 are coast fishes, and have a comparatively limited geographical 

 range, none extending from the northern temperate zone into 

 the southern. However, some, if not all the species of the 

 family Mylidbatidce, which includes the giants of this division 

 of Plagiostomes, have a claim of being included among the 

 Pelagic fishes, as they are frequently met with in the open 

 ocean at a great distance from the shore. It is probable 

 that the occurrence of such individuals in the open sea indi- 

 cates the neighbourhood of some bank or other comparatively 

 shallow locality. Many species are exclusively confined to 

 fresh water, and occur far inland, especially in tropical 

 America. 



The majority are oviparous. All have five pairs of gill- 

 openings. The number of known species is about the same 

 as that of Sharks, viz. 140. 



First Family — Peistid^. 



The snout is produced into an exceedingly long fiat lamina, 

 armed with a series of strong teeth along each edge (saw). 



1 This exception is a Eay obtained during the "Challenger" expedition, 

 and said to have been dredged in 565 fathoms. 



