134 



PALAEONTOLOGY. 



Family IV — Kaiid^. 



(Bays.) 



This family of cartilaginous fishes is first indicated in the 

 carboniferous period by the spine of Pleuracanthus (fig. 37) ; 

 but unequivocal evidences, sufficiently perfect to yield generic 

 characters, have been discovered in liassic (Squaloraia, Arth- 

 ropterus), oolitic (Spathohatis, Belemndbatis), cretaceous, and 

 tertiary formations : they chiefly consist of portions of the 

 numerous and many-jointed fin-rays, of defensive spines, 

 dermal tubercles, and most commonly of teeth. The peculiar 

 modifications of the dental system, presented by the eagle-rays 

 (Myliobatidw) are unequivocally shewn by fossils of the tertiary 

 formations, and have not been found in earlier strata. 



Fig. 53. 

 Jaws and teeth of an Eagle-Kay (Myliobates aquila). 



The teeth of the rays are in general more numerous and 

 much smaller than those of the sharks ; they have less 

 mobility, are more closely impacted, and in some cases are 

 laterally united together by fine sutures, so as to form a kind 

 of mosaic pavement on both the upper and lower jaws. The 

 Myliobates, or eagle-rays, which present the last-mentioned 



