CLASSIFICATION OF FISHES. 



463 



fish Molacanthus Pallasii (Pig. 4^7), which occur in the 

 JS"orth Atlantic. 



Fig. 426.— Orihagoriscue odlongtts, young, natural 

 Bize.— After Harting. 



Fig. 427.— JKflio- 

 canthus PaUasii, 

 half grown, natural 

 size. — After Put- 

 nam. 



CLASS rV.— PISCES. 



Aquatic Vertebrates with a movable lower jaw, a cartilaginous or 

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

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

 with a single ventricle and auricle. Mostly oviparous. 



Subclass I. Masmobranchii. — Skeleton cartilaginous ; skull without 

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

 openings ; no opercular bones ; tail heterocercal ; scales 

 placoid ; heart with a pulsating aortic bulb ; optic nerves 

 forming a chiasina ; intestine with a spiral valve ; both 

 oviparous and viviparous. 



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

 Order 2. Holocephali (Chimsera). 



Subclass II. Ganoidei. — 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 iu some respects lika 

 the bony fishes ; the living forms oviparous. 



Order 1. Ghondroganoidei (Acipenser). 

 Order 3. Dipnoi (Ceratodus, Lepidosiren). 



