NECTURUS MACULATUS. 407 
developed, but this point must be left for later investigation. It runs parallel to and very 
near the ventral quadrate process, and between the two there is thus formed a deep notch, 
at the bottom of which is seen the outer opening of the stylo-mastoid canal, that transmits 
the facialis nerve. This canal enters the pro-otic at its inner posterior margin, traverses 
almost its entire ventral wall and emerges by the notch just described. 
Internally the pro-otic is hollowed out for the reception of the anterior portion of the 
membranous labyrinth. At the junction of its internal and dorsal edges there is a canal 
for the transmission of the anterior vertical semi-circular canal, the ampulla of which les 
in a deep but narrow excavation at the apex of the cone. The anterior portion of the 
horizontal semi-circular canal is lodged in a groove upon the outer side of the internal 
excavation, just beneath the dorsal quadrate process. 
The pro-otic may come in contact with three bones, the actual condition varying 
with the age and degree of development. Its ventral surface is always overlapped by the 
parabasal, and is usually somewhat roughened over the region of contact. The parietal 
partly overlaps it dorsally. In large and mature specimens the anterior border of the 
operculum may touch it upon the ventral side. 
2. opistHoTic. This bone consists of a hollow cup, forming the posterior lateral 
angle of the skull, and investing the posterior part of the otic capsule. It is irregularly 
conical in shape, being somewhat flattened like a triangular pyramid, and thus presents 
externally for examina- 
Canal for 
horigental 
™ Semi-c Canal 
a 
\ 
tion three nearly flat sur- 
4 faces, a dorsal, a ventral, 
eo and an internal lateral, 
cele Canal py nae facing the occipital con- 
Pern: dyle. Of these surfaces, 
- : —" Mastoid : 
mastoid Position 05 Baers the dorsal is the flattest, 
Procers. Post. vert. . 
Pent Cicer: the ventral is quite con- 
VENTRAL DORSAL INTERNAL 
vex, and the internal lat- 
Fig. 9. Three views of the right opisthotic. 3. Taken from a specimen eral is marked by a deep 
smaller than in the case above. : . 
groove along which lies 
; the vagus group of cra- 
nial nerves. The apex of the cone or pyramid is directed backwards and a little out- 
wards, and terminates in a slightly elevated process, which may be called the mastoid for 
convenience, since it has some analogy with the mammalian process of that name. 
Internally the bone presents two canals, a large central depression, and in the bottom 
of the latter a still deeper recess into which the canals open. The two bony canals, which 
are excavated in the dorsal wall of the hollow cone, may be distinguished as the lateral 
