43 
side. They are free from each other, but attached to the 
outer walls of the branchial cavity. The lung is single (not 
paired as in Lepidosiren and Polypterus) but divided inter- 
nally into two symmetrical halves, each with about thirty 
compartments. A ductus pneumaticus communicates with 
two nasal openings within the mouth. The lung is probably 
inactive, and supplied with arterial blood so long as the fish 
is provided with abundance of pure water. When the water 
becomes thick and muddy by evaporation, it is likely that 
the animal fills its lung by swallowing air, and the lung 
then receives venous blood, returning it oxygenated to the 
heart by a separate pulmonary vein. 
Heart. — Instead of the two longitudinal valves which are 
contained within the hulhus arteriosus of Lepidosiren, Cera- 
todus has two or three transverse series, only one of which 
is fully developed. 
Without enlarging upon the many points of interest 
suggested by the description of Ceratodus, we must briefly 
point out its importance with reference to the definition of 
the order Ganoidei. 
Excepting only the presence of a true functional lung, all 
the characters of Lepidosiren and Frotopterus assimilate 
them to the class Pisces. In that class evidence of their 
special relationship to the Ganoids, Sharks, and Rays is 
afibrded by their muscular and contractile hidhus arteriosus, 
the valves in its interior, and the spiral valve of the intestine. 
It has already been seen how close is the agreement between 
Lepidosiren and the Ganoid Polypterus. They are indeed 
referred to difierent orders of fishes only on account of the 
diflerent function of the lungs, and the difierent position of 
the valves of the hulhus aoieriosus. Ceratodus now comes 
in to bridge the interval. It has the functional lung of 
Lepidosiren, the transverse arterial valves of Polypterus. 
Rearrangement of the whole group is thus rendered neces- 
