[31] THE OSTEOLOGY OF AMI A CALVA. 777 



become widened aud closed up at its dorsal aspect. An anatomical 

 reason for this closure, I believe, must be looked for in the development 

 of the hyomandibular in bony fishes. While in the Selachians this part 

 of the skeleton is a slender cartilaginous rod ; in osseous fishes it be- 

 comes more extensive, in conformity with the greater development and 

 diflt'erentiation of the muscular system of the maxillary apparatus, so as 

 to form a broad plate. Correlated with this, we also find the articular 

 facet for this bone in Teleosteans, extending so far as the postorbital 

 process, which extension anteriorily closes the hyomandibular cleft. 

 The relations of the main trunk of the facial nerve — the truncus hyoides 

 mandibularis — affords the strongest proof that this extension was in 

 anterior direction, or towards the anterior extremity of the body. In 

 Selachians this nerve passes close to the hyomandibular, coursing down- 

 wards in front of its anterior margin, while in the Teleosteans, in the 

 majority of forms, it perforates the hyomandibular bone in order to reach 

 the outer side. It requires no special demonstration to show that such 

 an apparent perforation of the bone could only have been accomplished 

 by its growth forwards, inclosing the nerve as it did so. At the same 

 time the hyomandibular cleft had to be necessarily closed up and trans- 

 formed into a blind recess in the gill cavity and with the same topo- 

 graphical relations with the labyrinth as we have described for it. 



At the base of the orbital region, in the interior of the skull, there is 

 a depression which is well defined both anteriorly and posteriorly, that 

 reminds us to some degree of the sella turcica of the higher vertebrates 

 (Plate II, Fig. 4). Posteriorly, this depression is continued beneath 

 the processes of the petrosal bone, already referred to, where it termi- 

 nates ; anteriorly it is bounded by a bar of cartilage, whicli contains an 

 osseous center at each lateral angle. At the base of this pit there is a 

 breach in the primoidal cranium, already mentioned, which is closed in 

 below by the parasphenoid. In the direction of the cavum cranii, speak- 

 ing in a more limited sense, this pit is entirely closed by a strong mem- 

 brane, which glistens like a tendon. This latter spans the space be- 

 tween the anterior sharp margin of the united and horizontal wings 

 of the ossa petrosa to the foremost cartilaginous bar. This membrane 

 extends far up the lateral walls of the skull, aud becomes attached 

 about half way up to a sharp bony crest that is developed downwards 

 and mesially from the ali- and orbito sphenoid (Fig. 7, KU^). The poste- 

 rior part of this upper extension of fascia ensheaths the trigeminal and 

 facial nerves near their points of exit from the skull; the anterior part 

 of this fascia is the membrane that closes the optic foramen. 



This fascia divides off another space, situated below and somewhat 

 laterally from the true cavity of the skull, which of course is intended 

 for the brain. The greater part of this space is filled in with the well- 

 known lymphoid fat tissue, found so extensively throughout the fishes, 

 that is also contained in the remaining part of the cavum cranii in 



■•^Dr. Sagemehl has failed to place these letters on his figure. — Trans. 



