249 
tiles these connective tissues become a definite membrane which may 
undergo more or less complete ossification as the processus descendens 
of the parietal bone, that bone and the remaining, unossified portions 
of the membrane, if any, bounding laterally a cavity, the cavum ep- 
iptericum, which I have already shown to be the homologue of some 
part of the trigemino-facialis chamber of Amia (Aruıs, 1914). 
Whenever, in amphibians and reptiles, and whether there be 
a cavum epiptericum or not, the trigeminus and facialis nerves per- 
forate the primary lateral wall of the chondrocranium by separate 
foramina, the cartilage that separates the two foramina must accor- 
dingly be the homologue of the cartilage that frequently separates 
the corresponding foramina in fishes, this cartilage, in the former 
animals, thus representing a part of the middle one of the three walls 
that I have here deseribed in fishes. That this cartilage (commissura 
praefacialis), bounding anteriorly the foramen faciale, can be prop- 
erly considered to represent a part of the commissura basicapsularis 
anterior of embryos, as Gaupp (1911) concludes that it does in the 
Sauropsida and in certain amphibians, would seem to me doubttul, 
for the corresponding cartilage in selachians evidently belongs to the 
side wall of the chondrocranium and not to the basal plate; though 
it may of course be an outgrowth of the latter plate. And it may be 
further added that the nervus abducens of Lacerta apparently pierces 
the crista sellaris of that animal, exactly as it does the corresponding 
prootie bridge in certain fishes, and not the basal plate of the chon- 
drocranıum, as Gaupp concludes. 
In Echidna, the cavum epiptericum of Gaupp’s (1908) deserip- 
tions has a mesial wall formed by what that author considers as a 
part of the dura mater, this membrane, so far as can be judged from 
the figures given, stretching across a space that extends from the 
hind edge of the meatus acusticus internus to the hind edge ot 
the ala orbitalis. The membrane passes along the internal surface of 
the taenia clino-orbitalis and extends upward from the floor of the 
cranial cavity to the internal surface of the commissura orbito- 
parietalis slightly dorsal to the free ventral edge of that cartilage. 
This membrane thus apparently corresponds to the membrane that, 
in certain specimens of Chlamydoselachus, closes the cerebral opening 
of the acustico-trigemino-facialis recess, the free, projecting, ventral 
edge of the commissura orbito-parietalis of Echidna then representing 
a remnant of the lateral wall of the recess of Chlamydoselachus and 
