185] OSTEOLOGY OF LORICATI —GUTBERLET IS 



Orbit. — The orbits are formed by the prefrontals, frontals, sphe- 

 notics, and suborbitals. They are large in Ophidon and the two are 

 separated by the partial partition of the orbitosphenoid which is carti- 

 laginous at its anterior portion and membranous at the posterior. At 

 the anterior end the partition comes in contact with the prefrontals in 

 the median line and at the posterior end it comes in contact with the 

 dorsal surface of the basisphenoid and the pedicle unites with the mem- 

 brane at the point of contact with the parasphenoid. Dorsal to the 

 basisphenoid the membranous partition divides, extends laterally and 

 unites on each side with the ventro-mesial edge of the alisphenoid and 

 the ventral processes of the frontal, thus forming the posterior wall of 

 the orbit and the anterior wall of the brain cavity. Directly dorsal to 

 the basisphenoid and slightly lateral to the median line there is an 

 opening through the membrane for the passage of the optic nerve. 

 Above the openings for the optic nerves in the median line against the 

 dorsal wall there is an opening for the olfactory nerves which pass 

 cephalad and divide, one passing on either side of the partition and 

 through the foramina in the prefrontal into the nasal sac. 



Brain Cavity. — The brain cavity in Ophidon is quite large 

 and extends from the posterior end of the orbit, from which it is sepa- 

 rated by the lateral partitions of the infraorbital membrane, to the post- 

 erior wall of the cranium. The cavity has a series of recesses on the 

 dorso-lateral walls for parts of the brain and on the ventral surface there 

 is a groove like cavity, the cavum sinus imparls, beneath the foramen mag- 

 num. On the floor of the brain case are two grooves which extend caudad 

 along the lateral walls of the basioccipital, parallel to the cavum sinus 

 imparls to the posterior end. The mesial edges of the exoccipitals overlap 

 and form saccular recesses of these grooves. Above each saccular recess 

 there is a cavity — the part of the labyrinth enclosing that portion of 

 the posterior semicircular canal which is located in the exoccipital. 

 Dorsal to this is a large recess in the epiotic which is for the rest of the 

 posterior semicircular canal. Antero-lateral to this recess in the pterotic 

 there is a large recess for the external semicircular canal, with two pits 

 in the ventral surface for the ampulla of this canal. One of these pits 

 is located in the prootic and the other in the pterotic. Anterior to this 

 recess there is another for the anterior semi-circular canal. This one is 

 divided into two parts by a flange-like partition which extends dorso- 

 cephalad from the floor of the cavity. The anterior part of the cavity 

 lies in the prootic and lodges the ampulla, while the posterior lies in 

 the sphenotic and lodges the body of the canal. Directly anterior to the 

 anterior semicircular canal there is another shallow cavity — the trige- 

 mino-facial chamber — which lies nearly in the line with the pituitary 

 opening between the basisphenoid and the prootics. The dorso-anterior 



