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 x'entral 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 lies 

 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 antei'ior border of the 

 operculum may touch it upon the ventral side. 



2. opiSTiiOTic. 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 ii*regularly 

 conical in shape, being somewhat flattened like a, triangular pyramid, and thus presents 



externally for examina- 

 tion three nearly flat sur- 

 faces, a dorsal, a ventral, 

 ..xExo /^ \ «^.. ... r«.f,..« l^lf'"^ and an internal lateral, 



facing the occipital con- 

 dyle. Of these surfaces, 

 the dorsal is the flattest, 

 the ventral is quite con- 

 vex, and the internal lat- 

 eral is marked by a deep 



VCNTRAL 



DOFkSAL 



Fig. 9. Three views of the right opi.sthotic. 

 smaller than in the ca.se above. 



INTf RNAL 

 X 3. Taken from a .specimen 



groove along; which lies 



the vagus group of cra- 

 nial nerves. The apex of the cone or pj'ramid is directed backwards and a little outr 

 wards, and terminates in a slightly elevated process, which may be called the ina.stoid 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 l)ony canals, which 

 are excavated in the dorsal wall of the hollow cone, may Ijc distinguished as the latei'al 



