266 EDWARD PHELPS ALLIS, JR 
In the adult Amia the efferent pseudobranchial artery gives 
off its ophthalmica magna branch as it traverses the orbital 
opening of the myodome, the artery and this branch thus both 
appearing to here lie dorsal to the trabecula. In 8-mm. and 
10-mm. embryos I, however, find the artery passing ventral to 
the trabecula and there falling into the internal carotid as it 
turns upward to pass between the trabeculae. From the artery 
so formed the arteria ophthalmica magna arises, and runs out- 
ward, dorsal to the trabecula, thus lying, in Amia, on the op- 
posite side of the trabecula to that in which it is shown by Dohrn 
in a 10-mm. embryo of the trout (Dohrn,’86, fig. 2). The 
development of these arteries and their relations to the trabee- 
ulae need further investigation. 
The prootic bridge, which forms the roof of the prootice por- 
tion of the myodome of the adult Amia, is of relatively late 
formation, for it is not found in a 40-mm. specimen. In a 
43-mm specimen it has been formed, and, as in Salmo, lies 
at a certain distance dorsal to the fenestra ventralis myo- 
domus and separated from the tip of the notochord by 
an open space, closed by membrane, which is the homologue 
of the fenestra basicranialis posterior of the Sauropsida. The 
saccus vasculosus lies, in this specimen, wholly anterior to the 
anterior edge of the prootic bridge, directly in line with it and 
embedded in the dorsal surface of loose stringy connective tis- 
sue which fills this posterior portion of the myodome. The 
recti externi, which, in the adult, extend to the hind end of the 
myodome, do not, at this age, extend posteriorly even as far as 
the hind end of the saccus vasculosus, having their origins ap- 
proximately in the transverse plane of the posterior opening of 
the trigemino-facialis chamber. 
From these conditions in Amia, it is evident that the pre- 
spinal and prootice portions of the normal teleostean myodome 
would arise if the cartilage which, in Amia, separates the myo- 
dome from the canals for the internal carotid and efferent pseu- 
dobranchial arteries were to be resorbed, leaving more or less 
developed membranes in its place. This cartilage is known to 
undergo resorption during the ontogenetic development of cer- 
