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 propose! 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, 7.¢., 
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 (hemal) arches 
are imbedded. The skull is cartilaginous. The extremity of 
the lower j 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 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. 
