210 EDWARD PHELPS ALLIS, JR. 
the myodome directly into the anterior end of the groove. The 
groove quite unquestionably lodged, in the fresh specimen, the 
anterior portion of the dorsal aorta, as it does in my 51-mm. speci- 
men, and it may accordingly be called the aortal groove, the term 
myodome being limited to that canal as decribed by Ridewood. 
The anterior portion of the floor of the myodome, as thus 
defined and limited, is formed by the parasphenoid, the ascending 
processes of which form the ventral portions of its side walls. 
The middle portion of the floor is probably formed by the syn- 
chrondosis, in the mid-ventral line, dorsal to the parasphenoid, 
of the ventral ends of the ventral processes of the prootics, for 
that is the condition in my 5l-mm. specimen, but, as I do not 
wish to destroy my one skull of the adult of this fish, I cannot 
definitely say that this is so. A short posterior portion of the 
floor is probably formed, like its anterior portion, by the para- 
sphenoid, for it is so formed in the 51l-mm. specimen. Slightly 
posterior to the sutural line between the hind edges of the pro- 
otics and the anterior end of the basioccipital, the parasphenoid 
separates into two diverging hind ends which extend poste- 
riorly a certain distance, there resting upon the ventral edges 
of the bounding walls of the aortal groove. 
The dorsal portions of the side walls of the anterior portion 
of the myodome and the entire side walls of its posterior portion 
are formed by the ventral processes of the prootics, which are 
overlapped externally by the lateral edges of the parasphenoid. 
The roof of the myodome is formed by the horizontal proc- 
esses of the prootics, the so-called prootic bridge or shelf, but 
whether these processes suturate with each other in the median 
line or are separated by a median line of cartilage, I cannot tell 
from my specimen for the reason above given. The prootic 
bridge is perforated near its anterior edge by a small median 
foramen, the so-called pituitary opening of the brain case of 
my descriptions of other fishes, the bridge thus having post- 
pituitary and prepituitary portions. 
The basisphenoid, which, as Ridewood says, has no verti- 
cally descending process, suturates posteriorly with the ante- 
rior edge of the prepituitary portion of the prootic bridge; later- 
