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 Class IV. FISHES. 



Order I. CARTILAGINOUS FISHES. Bones soft, 

 destitute of fibres ; sutures of the cranium indistinct. 



Tribe I. CHONDROPTERYGIOUS FISHES—Gills fixed. 



I. Lips Jilted to act as suckers. No pectoral or ventral Jins. 



a. Seven branchial apertures on each side, with corresponding ca- 

 vities. 



Petromyzon. 

 Ammocetes. 



a a. Two branchial apertures, with a ventral aspect, leading to six 

 cavities internally. 



Myxine. 



II. Lips unjit to act as suckers. 



a. Pectorals free. 



b. Mouth under the snout, which is abbreviated and unarmed. Eyes 

 lateral. 



c. With temporal orifices. 

 (d. With an anal fin. Two dorsal fins. 

 e. Teeth conical and pointed. 

 /. The first dorsal fin placed nearly above the pectorals. 

 Squalus. 

 Galeus. 

 ff. The first dorsal fin placed nearly above the ventrals. 

 Scyllium. 

 e e. Teeth blunt, closely set. 

 Mustelus. 

 d d. Destitute of an anal fin. 

 Spinax. 

 Scymnus. 

 c c. Destitute of temporal orifices. 

 Carcharias. 

 Lamna. 

 b b. Mouth terminal. Eyes with a dorsal aspect. 

 Squatina. 

 a a. Pectorals coalescing with the snout. 



b. Gill openings, five on each side. 



c. Tail fleshy, and of ordinary proportions. 



Torpedo. 

 c c. Tail hard and slender. 

 d. Tail with a long serrated spine. 



Trygon. 

 dd. Tail' destitute of the serrated spine. 

 Kaia. 

 b b. Gill openings single on each side. 

 Chimaera. 



Tribe II. — Gills free. 



Acipenser. 



.Order II. OSSEOUS FISHES. Bones hard, fibrous; su- 

 tures of the cranium distinct. 



