PADDLE FISHES. 763 
SUB-CLASS GANOIDEI. THE GANOID FISHES. 
Skeleton bony or cartilaginous ; optic nerves forming a chiasma; arterial bulb rhyth- 
mically contractile, provided with several rows of valves; intestine usually with a 
spiral valve; ventral fins, if present, abdominal ; tail more or less heterocercal. Of this 
important sub-class, few species are now extant, and these few vary widely from one 
another. Of the earlier fossil fishes, a very large proportion are ganoids (ganos, 
splendor, many of the species being provided with shining enamelled plates). 
ORDER 2. SELACHOSTOMI. THE PADDLE 
FISHES. 
No subopercle, preopercle, interopercle or maxillary bones; a single broad branchio- 
stegal; ventral fins abdominal, with an entire series of basilar segments; branchihyals 
cartilaginous; premaxillaries forming the border of the large mouth; snout dilated, 
prolonged ; skin smooth or nearly so; tail heterocercal. This order contains but a single 
family, Polyodontide. (Selachos, a shark; stoma, mouth.) 
1 AN CII IE IPO WOOD IN AY ICID) Ad, 5 AN eld) IPN IO) IB) Joo 
FISHES. 
Body elongate, fusiform, subterete ; skin smooth or with minute roughnesses; sides of 
the upturned part of tail with bony plates; mouth very wide, terminal bat overhung 
by the long snout, which is produced into a long and thin spatula-like process, reticulate 
above and below, thin and flexible at its edges; jaws and palate with minute 
deciduous teeth; no barbels; gill openings wide; opercle rudimentary, striate, pro- 
duced into along skinny flap; no tongue; spiracles present; air bladder large, com- 
municating with the csophagus; intestine with a well-developed spiral valve; stomach 
cecal, with a broad divided pyloric appendage; dorsal far back, between ventrals and 
anal; caudal with its lower lobe well-developed, nearly as long as the upper; pectoral 
fins large, inserted low; lateral line present. 
There are two species of this singular family known, representing two genera, Poly- 
odon from America and Psephurus from China, They are large shark-like fishes, living in 
fresh-waters, and feeding on mud and minute Crustacea. 
*Gill rakera long and fine, exceedingly numerous; upper caudal fulcra narrow, 15 to 
20 in number. : : : : ° ‘ : ° : POLYODON. 3. 
GENUS 3. POLYODON. Lacepede. 
Polyodon, LACEPEDE, Hist. Nat. des Poissons, i, 403, 1798. 
Spatularia, SHAW, General Zoology, v, 362, 1804. 
Platirostra, LESUEUR, Journ. Acad. Nat. Sci. Phila., i, 223, 1818 (adult without teeth). 
Type, Polyodon feuille, LACEPEDE, Polyodon folium, Auct. 
Etymology, polus, many ; odon, tooth. 
Polyodontide with each branchial arch furnished with a double series of very long 
setiform gill rakers, the two series being divided by a broad membrane; upper caudal 
fulcra not enlarged. American. 
