550 MULLER ON THE STRUCTURE OF THE GANOIDS, 
are absent in Lepisosteus as in Polypterus, nor is the choroid gland 
present. The brain had become spoilt. 
The ophthalmic nerves are distributed in both fishes as usual 
to the muscles of the eye, but in their origin Lepisosteus presents 
a very remarkable difference, which I always found the same in 
several specimens. The trochlear nerve and the motor oculi 
unite with branches of the trigeminus, 7. e. are twigs from 
branches of the trigeminus; at their origin from the brain they 
are perfectly separable, they then become enclosed in the trige- 
minus, from which they cannot be separated by dissection. The 
trigeminus leaves the skull through two foramina, a smaller 
branch through a distinct aperture in the ala parva, the remain- 
ing portion of the trunk through a foramen between the great 
and small wing. The first branch is then composed of two 
-roots from each trunk. ‘The trochlear nerve and the motor 
oculi are twigs of that branch of the trigeminus which passes 
through a separate foramen in the ala parva. The branch for 
the rectus superior, internus, inferior and the obliquus inferior, 
contains also the branches for the ciliary nerves. The abducens 
nerve however passes out with the posterior part of the trunk of 
the trigeminus through a foramen between the greater and 
lesser wing of the sphenoid bone. 
In Polypterus the nerves of the muscles of the eye are all per- 
fectly separate. The trochlear nerve first passes through a 
distinct foramen, the two other muscular nerves with the first 
branch of the trigeminus through another foramen. Moreover, 
a root of the main trunk of the trigeminus which comes out 
more posteriorly joins the first branch of the trigeminus. The 
foramen for the trochlearis and that for the first branch of the 
trigeminus, motor oculi and abducens, are placed in the descend- 
ing plate of the frontal bone, the foramen for the main trunk of 
the trigeminus between the frontal and ethmoid bones. 
The opercular branch of the trigeminus in Lepisosteus and 
Polypterus resembles that in the osseous fishes ; in Lepisosteus 
it separates from the trigeminus passing into a canal in the ala 
magna, perforates the temporal bone, then runs through a 
tract on the outer side of the preoperculum, and is prolonged 
on the inner side of the operculum. The accessory gills in 
Lepisosteus reccive branches from the glossopharyngeal nerve, 
which unites in both fishes with the opercular branch of the tri- 
geminus, and is distributed as usual. 
