GENERAL CHARACTER OF THE RAYS. l6Q 



the circumference ; but the mouth, nostrils, and branchial 

 apertures are on the under surface, and are completely 

 hid when the fish is laid on its belly : the latter organs 

 are generally five in number, arranged on the sides, and 

 are of the same form as in the sharks. The rays of the 

 Ads, like the rest of the skeleton, are cartilaginous, 

 straight, and furnished with numerous swellings or knots. 

 The mouth is small, and furnished with numerous small 

 blunt teeth, which are placed in rows, like paving stones 

 or mosaic, so as to completely cover the lips or edges 

 of the mouth : the eyes are protected by a nictitating 

 membrane or skin, which can at pleasure be drawn over 

 them like an eyelid, — a character which is common to 

 many of the sharks : at some distance above the eyes are 

 situated the nostrils, each appearing like a large and some- 

 what semilunar opening, edged with a reticulated skin, 

 and furnished internally with a great many laminated pro- 

 cesses, divided by a middle partition, and guarded by an 

 exterior valve : behind the eyes are the temporal orifices 

 or spiracles, communicating with the mouth and gills ; 

 these orifices are much larger than those of the sharks, 

 and often exceed the size of the eye ; and all these parts 

 taken together occupy a wide extent of surface. The young 

 are contained in oblong square capsules, of a horny sub- 

 stance, with a filament, more or less lengthened, at each 

 of the four corners. It would seem that the female has 

 the faculty of twisting these round the stems of marine 

 plants or corals, so as to secure the capsule from being 

 tossed about and drifted by the waves. These cases, 

 when the young have been exuded, are finally de- 

 tached, and are often cast upon the shore in considerable 

 numbers, when they are called sea purses by the common 

 people. 



(150.) Little is known of the natural history of these 

 singular fishes : inhabiting the depths of the ocean, they 

 elude the inquisitive eye of man ; and we can only form 

 a few conjectures by their general structure. We know 

 that the Pleuronectida >,, or true flat fish, lie concealed at 



