216 EDWARD PHELPS ALLIS, JR. 
musculi recti externi, with the dorsal aorta lying ventral to 
them. 
Proceeding anteriorly from this point to sections through the 
bases of the diverging hind ends of the parasphenoid (fig. 8), 
the bony bounding walls of the aortal groove are gradually re- 
placed by cartilage lined with thin plates of perichondrial bone 
which form parts of the basicccipital. Angles in this cartilage 
and bone now replace the two bony ridges, just described, in more 
posterior sections. The perichondrial bone then disappears, in 
the region of the hind end of the myodome, leaving the bounding 
walls of the groove entirely of cartilage, and slightly anterior 
to that point the remaining portions of the basioccipital also 
vanish. The notochord extends forward nearly to the hind end 
of the myodome, its anterior end lying dorsal to the bottom of 
the aortal groove and hence in the level of the roof of the myo- 
dome and not in that of its floor. In this region the aorta has 
separated into a lateral dorsal aorta on either side. 
Anterior to the bases of the diverging hind ends of the para- 
sphenoid, the aortal groove is closed ventrally by the latter 
bone, and, still lodging the musculi recti externi, becomes the 
hind end of the myodome. Except that the groove is here 
closed ventrally by the parasphenoid and that it lies in the pro- 
otic region, there is no line of demarcation between it and the 
open canal in the basioccipital region, and each broad ventral 
edge of the open groove, lying between the two angles above . 
described, is continued forward as the ventral edge of the lateral 
bounding wall of the myodomie canal. 
Proceeding forward in the sections, there is gradual ventral 
growth of the cartilaginous side walls of the myodomiec canal, 
this growth taking place between the two little angles above 
described. This gives rise to a flange of cartilage on either edge 
of the primitive groove, the flange projecting ventrally and 
slightly mesially beneath the level of the dorsolaterally pro- 
jecting basal portion of the lateral wall of the cranium, the base 
of that portion of the wall lying in the level of the ventral edge 
of the primitive aortal groove (fig. 7). Proceeding anteriorly, 
these flanges increase gradually in actual height, and appear to 
