CLASSIFICATION' OF FISHES. 



463 



fish", Molacanthus Pallasii (Fig. 437), which occur in the 

 North Atlantic. 



Fig. 426.— Orihago7'iseu8 oNongus, young, natural 

 size.— After Harting. 



Fig. ^i.—Molii- 

 canthus PoXlasii.. 

 half grown, natural 

 size. — After Pat- 

 nam. 



CLASS IV.— PISCES. 



Aqiicdic Yertebrates with a mmahle lower jaw, a cartilaginous or 

 osseoiia skeleton, with pnired and unpaired fins supported by fin rays ; 

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

 with a single mntricle and auricle. Mostly oviparous. 



Subclass I. Masmobranchii. — Skeleton cartilaginous ; skull without 

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

 openinga ; no opercular bones ; tail heterocercal ; scales . 

 placoid ; heart with a pulsating aortic bulb ; optic nerve* 

 forming a cliiasiua ; intestine with a spiral valve ; both 

 oviparous and viviparous. 



Order 1. Plagiostomi (Selaclie, Lamna, Baja). 

 Order 2. HolocepJi/ili (Chimaera). 



Bvbclass II. Oanaidei. — Skeleton cartilaginous or ossified ; skull with 

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

 ered l)y opercular bones ; skin usually with cycloid or gan- 

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

 young sorai-times with externnl gills ; cliiasma of the optie 

 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. Ghondroganoidei (Acipenser). 

 Order 8. Dipnoi (Ceratodus, LepidosirenV 



