MYODOME AND TRIGEMINO-FACIALIS CHAMBER 213 
(l. c., fig. 7). If, in those vertebrae that are known to have 
fused with the occipital region of the cranium of this fish, the 
deposit of bone between the aortal supports had been as re- 
stricted as it is in the first caudal vertebra, conditions similar 
to those actually found in Hyodon would have arisen. The pos- 
terior portion of the aortal groove of Hyodon is thus certainly 
enclosed between processes of vertebral origin, but whether these 
processes are the exact homologues of the haemal processes of 
the tail is open to some question, for there is marked want of 
accord in the descriptions of the formation of the latter. 
In Amia the aortal supports (haemal processes, Schauins- 
land) are said by both Hay and Schauinsland to be primarily 
cartilaginous, and to be simply differentiated parts of the bases 
of ventrolateral vertebral processes (lower arches). Posterior ° 
to the twenty-fourth vertebra, these supports are said by Hay 
to be developed apparently independently of the main mass 
of the ventrolateral vertebral processes (lower arches), and in 
the posterior region of the trunk they are said to be forced away 
from the notochord by bony deposits, and to each there become 
attached to the ventral surface of the remaining portion of the 
related ventrolateral process, which is then called a parapoph- 
ysis. In the tail region the aortal supports are said by Hay 
to entirely disappear, and this one statement, together with 
the several figures given, would lead one to suppose that it is 
the remaining portions only of the ventrolateral processes, the 
so-called parapophyses, that form the haemal arches of the tail. 
The descriptions are, however, not clear as to this. What Hay 
actually says is (95, p. 16): 
In the vertebrae of the tail the cartilages [aortal supports] are miss- 
ing. There,is, however, in my younger specimen, what seems to be 
vestiges of them in the first caudal vertebra. Nothing, however, can 
be more certain than that the lower arches of the trunk are bent down 
to form the arches of the tail, and that the aortal supports have 
nothing to do with the formation of the caudal haemal arches. 
On a later page he, however, says: ‘‘In the tail the halves of 
each lower arch have united at their distal ends, so as to enclose 
the blood vessels.” It may accordingly be that Hay con- 
