212 EDWARD PHELPS ALLIS, JR. 
are of membrane origin. The dorsolateral ridge of the process of 
the preceding figure has disappeared, but the ventromesial one is 
represented by a tall ridge of bone which bounds laterally on 
either side a deep aortal groove. The dorsolateral vertebral 
processes (upper arches, basidorsals) are here represented by 
two large blocks of cartilage in relation to which the exoccipi- 
tals are developed. 
Each exoccipital is perforated by two occipital nerves, the 
anterior one represented by a ventral root alone and the poste- 
rior one by both dorsal and ventral roots. Anterior to these 
two nerves a delicate ventral root arises from the medulla, but 
it: does not reach the internal surface of the cranial wall. 
On the ventral surface of the first free vertebra there is a 
slight depression, but no aortal groove, the space between the 
ventrolateral vertebral processes of opposite sides being com- 
pletely filled by bony deposits. 
Comparing these conditions with those in the adult Amia, it is 
evident that the ventromesial ridges of the ventrolateral cartilag- 
inous processes of Hyodon, bounding laterally the aortal groove, 
are the homologues of the little cartilaginous-processes on the ven- 
tral surface of the hind end of the basioccipital of Amia. There 
are in Amia, as is well known, two pairs of these little carti- 
laginous processes—called by Hay (95) aortal supports, by 
Schauinsland (05) haemal processes—and Schauinsland says 
that they are related to certain vertebrae that were said by 
Sagemehl (’83) to have fused with the hind end of the primor- 
dial cranium of this fish. Schauinsland further says that Sage- 
mehl found one pair of these processes, but that he himself finds 
two; but these two pairs had already been described by both 
me (’97) and Schreiner (02). The space between each pair of 
these processes is almost completely filled by bony deposit, the 
aortal groove thus being obliterated here, as it is also shown 
by Hay (95, fig. 1) to be in a transverse section of one of the 
anterior dorsal (trunk) vertebrae of a 12.5-em. specimen of 
this fish. In the last dorsal vertebra of this same specimen, 
Hay shows (l.c., fig. 6) this space not so completely filled by bony 
deposit, and in the first caudal vertebra it is even still less so 
