470 ELASMOBRANCHIl [CH. 



in fact in this group taken on the function of pumping water into 

 the pharynx, a duty which cannot be conveniently undertaken by 

 the mouth when this is burrowing in the mud at the bottom. Raid 

 is the common skate on both sides of the Atlantic : it has no caudal 

 fin but two dorsals. Torpedo is distinguished by a more elongated 

 body. The muscles on either side of the head are converted into 

 electric organs, consisting of batteries of vertical hexagonal tubes 

 filled with a clear gelatinous fluid, each tube representing a meta- 

 morphosed muscle-fibre. By means of these organs it can inflict 

 a severe shock on its enemies. Trygon is the sting-ray. In 

 it the tail is long and thin and the dorsal fin-rays are practically 

 absent, but at the spot where the tail merges into the body there is 

 a large recurved spine, at the base of which is a poison gland, so 

 that by a blow of the whip-like tail it can inflict a severe wound. 

 The pectoral fins are joined in front of the snout. Pristis is the 

 saw-fish. It has an immensely elongated rostrum, at the sides of 

 which large pointed teeth are set; the body is elongated, but it 

 shows all the essential features of the Batoidei. The teeth in the 

 mouth, like those of other Batoidei, are flattened, but the saw-fish 

 is an active predaceous fish and not a bottom feeder ; it is in fact 

 a Batoid which is assuming the habits of a Selachoid. Pmstis is 

 found both in the Mediterranean and Caribbean Seas and elsewhere. 

 In some of the extinct representatives of the family the upper jaw is 

 directly attached to the cranium behind the orbit. This variation 

 in the place of attachment indicates that the connection between 

 the two structures is secondary. 



The two most interesting fossil representatives of the Elasmo- 

 branchii are Cladoselache and Pleuracanthus whose fins are described 

 above (p. 4GO). 



Order II. Holocephali, 



The second order of Chondrichthyes, the Holocephali, differ from 

 Elasmobranchs chiefly in the skeleton ; in the viscera they resemble 

 them very closely. The Holocephali are distinguished by having the 

 upper jaw completely confluent with the cranium, a condition called 

 autostylic (Fig. 230) : the orbits are so deeply indented that the 

 brain is pressed back from between them, and their two cavities are 

 only separated by a vertical plate of cartilage, called the inter- 

 orbital septum. There is no spiracle and the last gill-cleft is also 

 closed. A fold of skin, called the operculum, extends back over 



