510 MULLER ON THE STRUCTURE OF THE GANOIDS, 
on the accessory gill as a peculiarity of the Sturgeon not existing 
in the other fish with an operculum and free gills, that fish was 
found to possess in addition a respiratory opercular gill, which 
was not then known to occur in any other fish having an oper- 
culum and free gills, nor in any Ganoid. It is also wanting in 
Polypterus, and I therefore had not, until that time, any suffi- 
cient ground for placing the Sturgeons and Ganoids together. 
Moreover, the Spatularie, which are inseparable from the Stur- 
geons, did not, in consequence of being unarmed, present any 
point of comparison with the Ganoids which were so very scaly. 
But when I had obtained an opportunity of examining Lepisos- 
teus, and still found in it these same peculiarities of a respiratory 
opercular gill, the position of the Sturgeon among the Ganoids 
was clear and decided ; and the peculiarity which I pointed out 
as existing in the Sturgeons only, of possessing an opercular 
gill, was now extended as a peculiarity naturally possessed by 
the Ganoids, but not occurring in true osseous fishes. 
In Lepisosteus the respiratory gill of the operculum is present 
as well as a pseudo-branchia. The statement made by Va- 
lentin *, in his account of my investigations regarding the false 
accessory gills or pseudo-branchiz of Lepisosteus, and considered 
by him as its internal and external accessory gills, is explained 
by the existence of a respiratory accessory gill as well as a 
pseudo-branchia. These two organs preserve the same relation 
as in the Sturgeons. I have proved their true importance by the 
investigation of their blood-vessels. The gill of the operculum 
of Lepisosteus is very considerable, and its upper end is joined 
at an acute angle to the pseudo-branchia, which is much smaller. 
Both organs, which as in the Sturgeons are of similar external 
structure, are in contact by their extremities, without adhering. 
The direction of the plates is opposite, at the point of contact. 
The muscular bulbus arteriosus, as in the Sturgeons and Poly- 
pterus, forms a very long trunk ; its muscular structure suddenly 
terminates just behind the spot where the artery begins to di- 
vide. The artery then divides into an anterior and a posterior 
branch. Two branches arise from the posterior portion on each 
side, the anterior of which is the artery of the gill of the second 
gill-arch; the posterior again divides into the arteries of the 
third and fourth gill-arches. The anterior portion of the ¢run- 
cus arteriosus proceeds forwards, then gives off on each side 
* Valentin, Repert. 1841, p. 137. 
