TYLOTOTRITON. 857 



element is a pointed process witli a foramen for the 5th nerve near its extremity. 

 There are two otlier foramina immediately above it, but they are frequently united 

 into one. They appear to be for the passage of blood-vessels as the roof of the 

 cranial cavity within, and is covered with a dense plexus. 



The exoccipital forms the wall of the cranium internal to the auditory capsule, 

 and a small part of the floor on each side of the parasphenoid. A narrow plate of 

 bone rises immediately behind the condyle, bending upwards, forwards and inwards 

 over the cavity, forming the superior margin of the foramen magnum. The base of 

 this arched process springs, as abeady stated, from the oixter wall of the fenestra 

 ovalis, so that only the narrow line along the posterior margin of the ridge is ex- 

 occipital, while all anterior to it superiorly is pro-otic. 



Each exoccipital, as a rule, has two articulating surfaces (fig. 25) for the 1st 

 vertebra, an external one for the lateral facet and an internal one for articulation 

 with the so-called odontoid process. The latter surface is a true articulation, 

 and it is confluent sometimes with the former, producing an elongated facet. 

 In cases where the odontoid process is not divided, each exoccipital carries a facet 

 and a half. The lateral facet is directed downwards, backwards and inwards, the 

 concavity being from without inwards, and the odontoid surface upwards and back- 

 wards with a prominent lower margin. 



The condyle is connected to the fenestra ovalis by a ridge, which, along with the 

 superior one, marking the limits of the pro-otic, encloses the foramen of the 8th pair 

 of nerves. There is a prominent notch in front of the odontoid articulation, which 

 receives a rounded flattened process on the encephalic aspect of the parasphenoid. 

 ■ Its posterior margin sometimes separates the odontoid articulations from each other. 

 The parasphenoid intervenes between the exoccipitals and forms the greater part of 

 the cranial floor. 



The two anterior of the three foramina on the wall of the cavity above, and 

 sHghtly before the parasphenoid notch of the exoccipital, lie in a depression common 

 to both ; but whilst the posterior of these two foramina passes directly into the 

 auditory chamber, the other pierces the substance of the opisthotic and opens below 

 at the base of the pterygoid. It appears to enclose the facial branch of the 7th 

 nerve, while the other transmits a nerve which also probably enters this simple 

 ear labrynth by the posterior foramen. 



The ^parasphenoid (fig. 33) is a long, rather flat, thin bone, stretching from the 

 posterior margin of the palatine canal to the inferior margin of the exoccipital 

 foramen, into which it enters. It has a slight constriction in its middle, correspond- 

 ing to the middle of the cranial cavity. The posterior portion is broader than the 

 anterior, and forms, as usual in this group of animals, the greater part of the floor of 

 the cavity posterior to the frontals. It is feebly convex below and concave above, with 

 a well-defined margin for the articulation of the orliito-sphenoid, and a broader one 

 posterior to this on which the exoccipital rests. Close to its posterior border, there is 

 a central rounded flattened nodule which fits into the precondyloid notch of the ex- 

 occipital, and sometimes sends a process between the odontoid facets of the two bones. 



M 5 



