460 ZOOLOGY. 
fish, Molacanthus Pallasti (Fig. 419), which occur in the 
North Atlantic. 
_ Fig. 419.—Mola 
\\ canthus Pallasii, 
half grown, natural 
Fig. 418.— Orthagoriscus oblongus, young, natural size.— After Put- 
size.—After Harting. 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. Hlasmobranchti.— 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 w 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 (Chimera). 
Subclass Il. Ganoidet.—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. Chondroganoidet (Acipenser). 
