462 ZOOLOGY. 



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 persistent, and as they were characteristic and 

 more numerous 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 term proposed by Wyman to designate the form 

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

 young garpike, 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 (ha3mal) 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 Geratodns 

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

 tary plates are very characteristic of the group. The narrow 

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

 many-jointed cartilaginous rod, to which are attached fine 

 fin-rays, supporting the thin edge of the fin. 



The spiral valve is present in the intestinal tract, ending 

 rather far from the cloaca, into which the oviducts and ure- 

 ters both open. There is a muscular conus arteriosus, and 

 the heart has, besides the right large auricle, a left smaller 

 one which receives the blood from the lungs, and a single 

 ventricle, as in Amphibians and most reptiles; they have 

 true nostrils. The lungs are like those of Amphibians, and 

 in addition they possess both internal and external gills, the 

 latter nearly or wholly aborted in the adult. 



