ORDERS OF FISHES. — 243 
and it has the body entirely covered with an armour of ganoid 
scales arranged in obliquely transverse rows. The jaws form a 
long narrow snout, armed with a double series of teeth, and the 
tail is heterocercal. The vertebral column is more perfectly 
ossified than in any other fish, the bodies of the vertebrae being 
convex in front and concave behind (“ ofzsthocewlous”). The 
Polypterus (fig. 129, A) inhabits the rivers Nile and Senegal, 
and is remarkable for the peculiar structure of the dorsal fin, 
which is broken up into a series of small detached portions, 
each composed of a single spine in front, with a soft fin attached 
to it behind. Some of the species of Polypterus are stated to 
possess extermaé gills when young, which they lose when grown 
AAG. 
- 
eT LAL LL 
ELT IP PLAS 
GEE 
LIF 
Fig. 129.-—Ganoid Fishes. A, Polypterus, a living Ganoid. B, Osteolepis, a fossil 
Ganoid (restored): a Pectoral fin: 4 Ventral fin ; c Anal fin; @ 2’ Dorsalgfins. * 
up, thus making an approach to the Amphibia. Many of ,the 
fossil Ganoids are more or less closely allied to the living Zepz- 
dosteus and Polypterus. 
Another great group of the Ganoid fishes is represented by 
the Sturgeons (Sturionide), in which the skeleton is always 
very imperfectly ossified, and the head, with more or less of the 
body, is protected by large ganoid plates, which are often united 
together at their edges by sutures. The true Sturgeons are 
chiefly found in the North Sea, the Caspian, and the Black Sea, 
and they are captured when ascending the great rivers for the 
purpose of spawning. The swim-bladder of the Sturgeons is 
