AND ON THE CLASSIFICATION OF FISH. Sel 
the gill-arteries.of the first arch, and is continued in the mid- 
die line as a thinner trunk. This azygous continuation of the 
trunk of the gill-artery passes out of the region of the gills 
of the gill-arches, and forms the trunk of the arteries of the oper- 
cular gills of the right and left side. After a course of half an 
inch it divides into a right and left branch, which direct them- 
selves towards the internal surface of the membrane of the gills, 
and run between the mucous membranes as far as the operculum 
and its gill, The membrane of the gill in Lepisosteus passes un- 
interruptedly in the form of a broad mantle from one side to the 
other, and is provided with an equally broad layer of transverse 
muscular fibres. 
In the Sturgeons, the continuation of the gill-artery to the first 
gill-arch also gives the artery of the gill of the operculum *. 
Hence the opercular gill of the Ganoids, like the true gills, con- 
tains dark red blood from the common arteries of the gills. 
The artery of the pseudo-branchia is exactly the reverse of 
this; it does not arise from the gill-artery, it belongs to the ar- 
terial system of the body, and thus conveys bright red blood to 
the pseudo-branchia totally different from a respiratory organ, 
just as the arteries do to all parts of the body. In Lepisosteus, 
as in other fishes, it is a continuation of the arterial system, 
a ramus opercularis, which supplies the bones and muscles of 
the operculum. In Lepisosteus it is seen through an aperture 
opening internally in the same place, as in the osseous fishes, 
I have never been able to trace its origin from the first gill-vein, 
which I have proved to occur in other fishes, from not having 
specimens, but there is no doubt that its relations are the 
same. 
The Sturgeons are distinguished from all other osseous fishes 
by their pseudo-branchia forming, as in the Plagiostomi, a rete 
mirabile caroticum for the eye and brain, whilst in all other 
osseous fishes a rete mirabile ophthalmicum alone exists. For 
‘reasons comprised in what I have stated above, the same may 
be imagined to occur in Lepisosteus. I must leave this question 
undecided until new specimens can be obtained. 
The existence of an accessory opercular gill is incompatible 
with the nature of an osseous fish—it is a character of the Ga- 
noids; but it is not necessarily peculiar to them. In the naked 
Spatularie most nearly related to the Sturgeons, as in Planirostra 
* See Anatomie der Myxinoiden, 3'° Fortsetzung. 
