CLASSIFICATION OF FISHES. 



463 



fish, Molacantlms Pallasii (Fig. 427), which occur in the 

 North Atlantic. 



Fig. 42G.— Oi'ihagorisciis ohlongus, young, natural 

 size.— After Harting. 



Fig. ^1.— Mola- 

 cantlms Pallcmi, 

 half grown, natural 

 size. — After Put- 

 nam. 



CLASS III.— PISCES. 



Aquatic Vertebrates with a movable lower jaw. a cartilaginous or 

 osseous skeleton, icith paired and unpaired fins supported by fin rays ; 

 no sternum; usually covered with scales; breathing by gills. Heart 

 with a single nertiricle and auricle. Mostly oviparous. 



Subclass I. Elasndbranchii. — Skeleton cartilaginous ; skull without 

 membraue bones, five to seven pairs of gill-sacs and gill- 

 openings ; no opercular bones ; tail lieterocercal ; scales 

 placoid ; heart with a pulsating aortic bulb ; optic nerves 

 forming a chiasma ; intestine with a spiral valve ; both 

 oviparous and viviparous. 



Order 1. Plagiostomi (Selache, Lamna, Kaia). 

 Order 3. Holocephali (Chinisera). 



Subclass 11. Oanoidei. — Skeleton cartilaginous or ossified ; skull with 

 plate-like membrane bones ; one pair of gill-openings cov- 

 ered by opercular bones ; skin usually with cycloid or gan- 

 oid scales ; air-bladder with a pneumatic duct ; embryos or 

 young sometimes with external gills ; chiasma of the optic 

 nerves ; intestine with a spiral valve ; development, so far 

 as known, much as in the sharks, and in some respects like 

 the bony fishes ; the living forms oviparous. 



Order 1. Ohondroganoidei ( Acipenser). 

 Order 3. Dipnoi (Ceratodus, Lepidosiren). 



