214 EDWARD PHELPS ALLIS, JR. 
sidered the haemal arches to be formed by the entire ventro- 
lateral processes, and this is what Schauinsland (’05) says of 
these arches in all fishes. In Laemargus, Schauinsland even 
shows (l. c., p. 411) the aortal supports (his haemal arches) pro- 
jecting mesially from the mesial surfaces of the entire ventro- 
lateral processes and partly separating the haemal canal into 
dorsal and ventral compartments which lodge, respectively, 
the aorta and the caudal vein. 
In Polypterus the haemal arches have, as described by Bud- 
gett (02), a totally different origin from that above set forth. 
In a 30-mm. specimen of this fish Budgett finds three distinctly 
separate series of cartilaginous vertebral processes, one dorsal, 
one lateral, and the other ventral. The lateral processes bear 
the upper ribs, which have the positions of the ribs in the Se- 
lachi. The ventral processes bear the lower ribs, which have 
the position of the ribs in the Teleostei. It is said that, in the 
caudal region, ‘‘the lateral series of cartilages are not found, while 
the ventral cartilages, though retaining their position, become 
the greatly enlarged haemal arches.” These latter arches are 
thus here formed by processes that are certainly not the homo- 
logues of the so-called lower arches of the Selachii. In the 
trunk region of specimens of Polypterus older than the 30-mm. 
one, the ventral processes are said to be forced away from the 
notochord by bony deposits formed in relation to the lateral 
processes and these bony deposits are shown, in Budgett’s 
figures, forming so-called aortal supports on either side of the 
aorta. The ventral processes thus forced away from the 
notochord are then found as blocks of cartilage in the bases of 
the ventral ribs at some distance from the notochord and loosely 
attached to the under sides of the lateral processes; these ven- 
tral processes of this fish thus strikingly resembling the aortal 
supports of Hay’s descriptions of Amia. 
The aortal supports and haemal arches, as those terms are 
employed by English authors, may thus be of different origin 
in different fishes, but, whatever their origins and homologies 
may be, the lateral walls of the aortal groove of Hyodon, in the 
posterior basioccipital region here under consideration, are quite 
