512 MULLER ON THE STRUCTURE OF THE GANOIDS, 
edentula, Raffinesque, I find no opercular gill, but merely a 
pseudo-branchia concealed in their blowing cavity, which oc- 
cupies the same position as the pseudo-branchiz in the Stur- 
geons. Polypterus bears the same relation to Lepisosteus as the 
Planirostre do to the Sturgeons. Like Planirostra, Polypterus 
possesses no opercular gill, and even the pseudo-branchia has 
here disappeared, only the blowing cavity has remained, in which 
I can recognise no trace of this organ. The occurrence of the 
pseudo-branchia is also subject to similar variations in the Pla- 
grostomi: for I have not found it in several genera, as Scymnus, 
in which however at an early period of foetal existence I have 
seen it in this situation*. The same applies to the blowing- 
hole. They are peculiar to most of the Plagiostomi, and, ac- 
cording to the above detailed observations, perhaps to all with- 
out exception in a foetal state; but in the adult state they are 
wanting in the genera Carcharias and Sphyrna. The same re- 
lations are repeated in the Ganoids. The Sturgeons have blowing- 
holes: in the nearest related genus, Scaphirhynchus, Heck., the 
blowing-hole has disappeared. It is present however in the 
Spatularie: in Planirostra edentula it is a small aperture, equi- 
distant from the eye and from the angle of the mouth. The 
Polypterus, as is well known, also possesses blowing-holes, but 
they are absent in Lepisosteus. Hence as regards the opercular 
gill, the pseudo-branchia and the ejecting-hole, almost all the 
combinations which are logically possible occur among the 
Ganoidei :— 
1. Opercular gill, pseudo-branchia and blowing-hole—Aci- 
penser. 
2. Opercular gill and pseudo-branchia without blowing-hole 
—Lepisosteus. 
3. Opercular gill without pseudo-branchia and without the 
blowing-hole—Scaphirhynchus. 
4, Pseudo-branchia without opercular gill but with blowing- 
hole—Planirostra. 
5. Blowing-hole without opercular gill and without pseudo- 
branchia—Polypterus. 
The presence of the blowing-hole is no absolute’ character of 
the Ganoids, for Lepisosteus forms an exception even among those 
now in existence, but the negation of this character is absolute 
* Vergl. Anat. der Myxinoiden, 3 Fortsetzung. dbhandl. d. Akademie d. 
Wissenschaft., 1840, p. 252. 
