AND ON THE CLASSIFICATION OF FISH. 547 
facialis of Lepisosteus arises on each side as a branch of the gill- 
vein of the first of the four gills, and passes on each side through 
a separate opening in the great wing of the sphenoid bone into 
the temporal fossa, to ramify in the external and anterior parts 
of the head. Lepisosteus differs from every other fish in pos- 
sessing pterygoid processes (formed from the base and ala magna 
of the sphenoid bone) and the articulation of the pterygoid bones 
to these processes, of which I know no other example either 
among the Ganoids or any other fishes. On the inner side of 
this articulation the pterygoid process is separated from the ba- 
silar process of the sphenoid bone by an imperfect canal. The 
internal carotid coming from the pseudo-branchia entered it 
inferiorly to turn upwards, and above this fissure the internal ca- 
rotid immediately enters a foramen leading into the cranial cavity. 
Hence, on injecting the pseudo-branchial vein with mercury, the 
vessels within the cranial cavity become filled. 
Polypterus has a single internal carotid, which arises from the 
union of the gill-veins and perforates the centre of the base 
of the occipital bone in a very peculiar manner. 
The arteries of the cellular swimming-bladder of Lepisosteus 
arise from the aorta in great number; the veins empty them- 
selves into the two subvertebral veins. The cellular swimming- 
bladder is therefore in this instance not of the nature of a 
lung. 
In Polypterus the arteries of tine swimming-bladders arise from 
the last gill-vein of each side at about the middle of the gill- 
arches which have grown to the gill-apertures. The veins of 
the swimming-bladders proceed to the middle vena cava, which 
also receives the hepatic veins. This peculiar vena cava impar, 
which is distinct from the two subvertebral veins, forms a con- 
‘siderable trunk from the posterior extremity of the abdominal 
eavity anterior to the anus, where it communicates with the 
_subyertebral veins and the caudal vein; at the posterior extre- 
amity of the right large swimming-bladder, which extends to the 
‘anus, it passes to the posterior extremity of the swimming- 
bladder anterior to it, and accompanies it, lying between it and 
the right lobe of the liver, which is very long, as far as the dia- 
phragm. It receives a great number of oblique branches from 
the right swimming-bladder, and finally, the main venous trunk 
of the right and left swimming-bladders. 
_ The swimming-bladders in Polypterus are completely sur- 
VOL. IV. PART XVI. 2R 
