280 EDWARD PHELPS ALLIS, JR. 
embryos of Acanthias, between the trabeculae and parachordals 
and primarily independent of those cartilages, thus correspond- 
ing to the hind ends of the trabeculae of Sewertzoff’s descriptions. 
and apparently also to the median, ventro-anteriorly directed 
process of the parachordal of Baumgartner’s description. 
The conditions in these fishes thus show that chrondification 
has taken place to such an extent in the prootic and subpituitary 
regions that the dorsal compartment of the teleostean myodome 
has been reduced, either to canals traversed by the pituitary 
veins or to some part of a deeper, posterior portion of the pituitary 
fossa of the chondrocranium. The remainder of the deeper 
portion of the fossa represents an anterior extension of the dorsal 
myodomie cavity which has been developed in some relation 
to the enclosure of the internal carotid arteries in it. The sub- 
dural canals traversed by those arteries after they leave this 
subpituitary space evidently form anterior prolongations of it, 
and were they to be added to it, and the pituitary fossa reduced 
to the proportions in Ceratodus and higher vertebrates, the 
arteries would traverse a peripituitary space separated from the 
cavum cerebrale cranii by the dura mater. The conditions 
here thus seem to indicate that the fenestra hypophyseos of 
these fishes is the homologue of the fenestra interparachordalis 
of the Holostei and Teleostei, and not of the fenestra hypoph- 
yseos of those fishes. The foramina carotica of Amia and 
the Selachii are then not homologous. 
No ventral myodomiec cavity is found in these fishes, except 
as it may be represented in a part of the canals traversed by the 
internal carotid arteries. The cross-commissure between these 
arteries has a position which suggests that it may have been 
utilized, in the Teleostei, to form the cross-commissure between 
the efferent pseudobranchial arteries. 
In certain of these fishes, a canal in the lateral wall of the chon- 
drocranium, traversed by the vena jugularis, represents, as in 
Polypterus, a pars jugularis of a trigemino-facialis chamber 
(Allis, ’14b). 
In the Batoidei, the pituitary fossa, as shown in Gegenbaur’s 
figures, is but slightly developed, but as he says that a canalis 
