37 
vertebral column notocbordal. The limbs are filiform fine, 
supported by jointed, cartilaginous rods. Both jaws are 
armed with corresponding dental plates, which are provided 
with cusps and cutting ridges. The intestine has a spiral 
valve. There is a double air-bladder, communicating by 
means of a duct and glottis with the haemal side of the 
oesophagus. The gills are free. The hulhus arteriosus has 
two longitudinal valves, and there is a second (left) auricle, 
receiving blood from the air-bladder, and incompletely 
separated from the right auricle by a reticulate septum. 
There are two blind nasal sacs, each with two openings. 
The optic nerves are united by a commissure. The oviducts 
are distinct, and the ventral fins abdominal. The body is 
eel-shaped, and covered with cycloid scales. 
The African genus (Protopterus) difiers from its ally 
Lepidosiren, the Mud-fish of the Amazons, in the following 
particulars : — The fins have a marginal fringe of numerous 
minute rays. Eudimentary external giUs are present in the 
adult. There are six branchial arches, instead of five, as in 
Lepidosiren. 
To all except those who may choose to suppose that 
Protopterus and the river Gambia were made simultaneously, 
each for the other, it will be apparent that the peculiarities 
of the respiratory organs of these fishes depend upon the 
peculiarities of their environment. Lepidosiren and Pro- 
topterus must, on any other hypothesis, be fishes of other 
than the existing dipnous type, which have been specially 
modified to suit their external conditions. It may seem 
hazardous to conjecture what the primaeval type may have 
been, but it is important to notice that most of the organs 
not immediately related to respiration conform to Muller's 
definition of Ganoids. The optic nerves form the same 
kind of commissure, the intestine has a spiral valve, the 
branchiae are free, the ventral fins are abdominal The only 
