THE ELASMOBRAKTCHII. 131 



The Rays usually have somewhat obtusely-pointed teeth, but 

 in Myliobates, the middle teeth have transversely elongated, 

 and the lateral ones hexagonal, flat crowns, and the various 

 teeth are fitted closely by their edges into a pavement. In 

 Aetohatis only the middle transversely elongated teeth re- 

 main. In the Sharks and Rays the teeth are developed from 

 papillae, or ridges, situated at the bottom of a deep fold 

 within the mucous membrane of the jaw. The teeth come 

 to the edge of the jaw, and, as they are torn away or worn 

 down by use, they are replaced by others, developed, in suc- 

 cessive rows, from the bottom of the groove. No such 

 successive development takes place in the Ghimcera. 



As in other fishes, there are no salivary glands. The 

 wide oesophagus leads into a stomach which is usually 

 spacious and sac-like, but sometimes, as in Chimcera, may 

 be hardly distinct from the rest of the alimentary canal. 

 No diverticulum filled with air, and constituting a swim- 

 ming-bladder, as in Ganoid and many Teleostean fishes, is 

 connected with either the cesophagus, or the stomach, 

 though a rudiment of this structure has lately been dis- 

 covered in some Elasmobranchs. 



The intestine is short, and usually commences by a 

 dilatation separated from the stomach by a pyloric valve. 

 This duodenal segment of the intestine is usually known as 

 the Bursa Entiana. It receives the hepatic and pancreatic 

 ducts, and, in the foetus, the vitelline duct. Beyond this 

 part, the absorptive area of the mucous membrane of the 

 small intestines is increased by the production of that 

 membrane into a fold, the so-called spiral valve, the fixed 

 edge of which usually runs spirally along the wall of the 

 intestine. In some sharks {CarcJiarias, Galeocerdo) the fixed 

 edge of the fold runs straight and parallel with the axis of 

 the intestine, and the fold is rolled up upon itself into a 

 cylindrical spiral. 



The short rectum terminates in the front part of a cloaca, 

 which is common to it and the ducta of the renal and the 

 reproductive organs. The peritoneal csLvitj communicates 

 with that of the pericardium in front, and, behind, opens 



