PADDLE FISHES. 763 



SUB-CLASS aANOIDEI. THE GANOID FISHES. 



Skeleton bony or cartilaginous ; optic nerves forming a chiasma ; artsrinl bulb rhyth- 

 mically contractile, provided with several rows of valves ; ii!t<=Btine usually with a 

 spiral valve ; ventral fins, if present, abdominal ; tail more or less heterocerc?.!. 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 ; branohihyals 

 cartilaginous ; premaxilUries forming the border of the large mouth ; snout dilated, 

 prolonged ; skin smooth or nearly so ; tail heterooercal. This order contains but a single 

 family, Polyodonlidce. {Selaohos, a shark; etovia, mouth.) 



FAMILY II. POLYODONTID^. THE PADDLE 



PISHES. 



Body elongate, fasiform, 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 apatula-liko process, reticulate 

 above and below, thin and flexible at its edges; jaws and palate with minute 

 deciduous teeth; no barbels; gill openings wide; operole rudimentary, striate, pro- 

 duced into along skinny flap; no tongue; spiracles present; air bladder large, com- 

 municating with the oesophagus ; intestine with a well-developed spiral valve ; stomach 

 ceecal, with a broad divided pyloric appendage ; dorsal far back, between veutrals 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, Foly- 

 odon from America and Paephuns from China. They are large shark-like fishes, living la 

 fresh-waters, and feeding on mud and minute Crustacea. 



*Gill rakers long and fine, exceedingly numerous ; upper caudal falcra 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. 



Platirosira, I^eSubur, Journ. Acad. Nat. Sol. Phila., i, 223, 1818 (adult without teeth). 



Type, Polyodoyifeuille, liA.CETi^DE, Polyodon folium, Auct. 



Etymology, polus, many ; odon, tooth. 



Polyodontidce with each branchial arch famished with a double series of very long 

 setiform gill rakers, the two series being divided by a broad membrane ; upper caudal 

 fnlcra not enlarged. American. 



