240 EDWARD PHELPS ALLIS, JR. 
says (91, p. 516) that this relation to the aorta at once suggests 
a lower vertebral arch. He, however, says that he finds weighty 
reasons against the assumption that it is such an arch. One 
of these reasons is that, excepting in this region of the Cypri- 
nidae and in the tail region of all fishes, the lower arches always 
enclose the body cavity, and not simply the aorta. A second 
reason is that he himself finds, in embryos of Chondrostomus 
nasus, the pharyngeal process not preformed in cartilage, as 
the lower arches always are. Sagemehl accordingly concludes 
that the pharyngeal process of the Cyprinidae is not a lower 
vertebral arch, and he considers it to be a bone formed by 
the fusion of pharyngeal bones of dermal origin with another 
bone formed by the ossification of a ligament which, in the 
Characinidae, extends from the hind end of the basis cranii to 
the swim-bladder, embracing the aorta in its course. 
In a 57-mm. specimen of Catostomus occidentalis I find the 
phyaryngeal process formed by two ventrally projecting lon- 
gitudinal flanges of bone which arise from a layer of bone sur- 
rounding the notochord, and, diverging slightly and straddling 
the aorta, abut against and fuse with the dorsal surface of a 
curved and porous plate which les parallel to the dorsal sur- 
face of the pharynx (fig. 29). The aorta is thus enclosed in 
a canal that corresponds strictly to the aortal groove of Hy- 
odon, except in that it is closed ventrally by the formation of a 
horizontal floor across its outer edges, and if the one is of verte- 
bral origin, as I consider it to be, the other certainly also is. 
Whether the floor of the canal has been developed in primary 
continuity with its lateral walls, or as an independent dermal 
formation, as Sagemehl suggests, cannot be told from my sec- 
tions. The lateral walls of the canal are prolonged anteriorly 
beyond its floor, and the aorta there lies (fig. 28) in an open 
groove similar to that of Hyodon, the lateral walls of the 
groove gradually diminishing in height and vanishing approx- 
imately in the level of the anterior end of the persisting noto- 
chord. Anterior to the point where the vacuolated contents of 
the notochord can last be recognized in the sections, the noto- 
chorda_ space still continues a certain distance, and in sections 
