4GO 



ZOOLOGY. 



fish, Molacantlms Pallasii (Fig. 419), which occur in the 

 North Atlantic. 



Fig. 418. Orthagoriscus oblongus, young, natural 

 size. After Harting. 



Pig. 419. Mole* 

 canthus Pallaxii, 

 half grown, natural 

 size. After Put- 

 nam. 



CLASS IV. 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. Elasmobranchii. 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 chiasma ; intestine with a spiral valve ; both 

 oviparous and viviparous. 



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

 Order 2. Holocephali (Chiniaera). 



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 in some respects like 

 the bony fishes ; the living forms oviparous. 



Order 1. Chondroganoidei (Acipenser). 



