428 ZOOLOGY. 



The singular spoon-bill, Polyodon folium Lacepfede, is five 

 feet long ; it is smooth-skinned and has a snout one- third as 

 long as the body, and spatulate, with thin edges. It has a 

 very wide mouth with minute teeth, and lives on small 

 Crustacea. It abounds in the Mississippi, and its larger 

 tributaries. 



Order 3. Dipyioi. — The lung fishes are so called from the 

 fact that often being in pools and streams liable to dry up, 

 they breathe air directly, having true lungs, like those of 

 Amphibians, as well as gills. From the nature of their lungs 

 and heart, the Dipnoans are quite different from all other 

 fishes, anticipating in nature the coming of Amphibians, 

 while on the other hand the notochord and sheath is persist- 

 ent, and as they were characteristic and more numerou^s in 

 Devonian times, they may be said to be a prematurative 

 type. 



The body of the Dipnoans is somewhat eel-shaped, though 

 not very long in proportion to its thickness, and is covered 

 with cycloid scales. The pectoral and ventral fins are long, 

 narrow, and pointed, and there is a long caudal fin which is 

 protocercal, a terra proposed by Wyman to designate the form 

 of the caudal fin of embryo sharks. In fact, the tail of the 

 young garf Jke, as of embryo Teleosts or bony fishes, is at 

 first protocercal, afterwards being heterocercal in adult 

 Ganoids, such as the garpike, and in the embryo and 

 early free stage of most bony fishes ; the tail in the latter 

 becoming finally homocercal or equal-lobed. Thus the 

 tail of the Dipnoans may be said to be embryonic, i.e., 

 protocercal. 



The spinal column is represented by a simple notochord and 

 sheath ; within the latter the basal ends of the bony neural 

 arches and ribs, and near the tail the lower (haemal) arches 

 are imbedded. The skull is cartilaginous. The extremity of 

 the lower jaws supports large tooth-like plates (dentary plates) 

 which shut in between the few palatine teeth ; in Ceratodus 

 these plates are single, and in all Dipnoans these single den- 

 tary plates are very characteristic of the group. The nacrow 

 pectoral and ventral fins are supported by a single, median, 



