ORDERS OF FISHES. 



163 



Fig. 530. — Batides. Raia vw^rginata, one of tlie 

 Skates. Reduced cue-sixth. (After Gosse.) 



flattened bodies of the Eays, however, must be carefully dis- 

 tinguished from those of the Flat-fishes (Pleuronectidce). In 

 the former, the flat surfaces 

 of the body are truly the 

 dorsal and ventral surfaces. 

 In the latter, as before re- 

 marked, the body is flat- 

 tened, not from above down- 

 wards, but from side to side, 

 and the head is so twisted 

 that both eyes are brought 

 to one side of the body. 



The tail in the Eays is 

 long and slender, usually 

 armed with 'spines, and gen- 

 erally with two or three fins 

 (the homologues of the dor- 

 sal fins). The mouth is 

 paved with flat teeth of a 

 more or less rhomboidal 



shape. The integument is often furnished with placoid 

 structures of a peculiar shape consisting of oval or rounded 

 osseous discs, from the centre of each of which springs a 

 curved spine of dentine. The tail also is sometimes armed 

 with a doubly-serrated defensive spine. 



The Eays are known in the fossil condition by their flat 

 crushing teeth mainly, but also by their fin-rays, bony discs, 

 and defensive spines. The earliest trace of the Eays is found 

 in the Carboniferous rocks, where occurs the doubly-serrated 

 spine wliich is referred to the genus Pleuracanthus (fig. 522, 

 1). In this singular form, however, the spine seems to have 

 been inserted at the back of the head, instead of in the tail, 

 as in the living Sting-rays ; and it is not certain that the 

 genus is not rather truly Selachian in its affinities. Another 

 ancient and remarkable form is Janassa ( = Climaxodus), of 

 the Carboniferous and Permian, which forms a connecting 

 link between the Eays and the Cestracionts, though seem- 

 ingly really referable to the former. In this genus, the mouth 

 is furnished with ovate teeth (fig. 531), arranged in slightly 



