MYODOME AND TRIGEMINO-FACIALIS CHAMBER 251 
In my 40-mm. specimen of Gasterosteus the internal caro- 
tid and efferent pseudobranchial arteries of either side perfor- 
ate the base of the ascending process of the parasphenoid 
through a single foramen, and enter the space beneath the trans- 
verse ridge on the parasphenoid. There the pseudobranchial 
artery is connected by a cross-commissure with its fellow of the 
opposite side, and then runs forward into the orbit as the ar- 
teria ophthalmica magna. The internal carotid gives off, before 
entering its foramen, its orbitonasal branch, which traverses 
the foramen with the internal carotid and efferent pseudo- 
branchial arteries, and then runs forward along the floor of the 
myodome to enter the orbit. The internal carotid, after giving 
off this branch and having entered the space beneath the trans- 
verse ridge on the parasphenoid, turns upward in the median 
vertical myodomic membrane, and, while in that membrane, 
anastomoses with its fellow of the opposite side. It then sepa- 
rates from its fellow and, while still in the membrane, divides 
into two parts, one of which at once enters the cavum cerebrale 
cranli, and is the posterior cerebral artery. The other part 
runs forward in the thick median portion of the membranous 
floor of the cavum cerebrale cranii, and, issuing beneath it, sends 
two branches to the eyeball, one of them accompaying the nervus 
opticus. The remainder of the artery then enters the cavum 
cerebrale crani through the foramen opticum, and is the ante- 
rior cerebral artery. No positive and definite connection between 
the anterior and posterior cerebral arteries was seen, the ante- 
rior branch of the latter artery, found in the other fishes, not 
occurring here. 
The ramus palatinus facialis arises from the trigemino-facialis 
ganglionic complex, and passing lateral and then ventral to the 
vena jugularis, runs ventromesially along the internal surface 
of the prootic bone and perforates the dorso-anterior portion 
of the transverse ridge on the parasphenoid to enter the space 
beneath it and then to escape into the orbit. This nerve, in this 
fish, thus lies lateral to the vena jugularis, while in all others 
in which it was traced (Hyodon, Scomber, Scorpaena, Cottus, 
Catostomus, and Amia) it lies mesial to that vein. This is, how- 
