NECTURUS 87 



nium are two pairs of large membrane bones, the parietals and 

 frontals. The parietals extend from the hinder end of the skull 

 forward; the frontals overlap the anterior projections of the 

 parietals and extend almost to'the forward end of the skull. 



The ventral portion of the cranium proper consists of a single 

 very large membrane bone, the parasphenoid, or parabasal, which 

 extends from the hinder almost to the forward end of the skull 

 and forms the roof of the mouth. 



Three pairs of special-sense capsules are present,— the auditory, 

 optic, and nasal. 



The nasal capsules occupy the forward end of the skull. Each 

 capsule is an elongated space which opens to the outside by means 

 of the anterior and the posterior nares. The two anterior nares 

 are situated at the forward end of the skull, with a wide space 

 between them ; the posterior nares are on the under surface, at the 

 hinder end of the capsule, and open into the mouth. The nasal 

 capsules consist largely of cartilage. The only bones which belong 

 to them are a pair of large membrane bones, the vomers, which 

 form most of their ventral surface. These bones overlap the for- 

 ward end of the parasphenoid and bear teeth along their anterior 

 edge ; between them, in the midventral area, will be seen an inter- 

 nasal plate of cartilage. The roof and sides of the nasal capsules 

 are formed by a delicate, fenestrated cartilage which is covered 

 above by the frontal bones. 



The optic capsules are a pair of very delicate, ring-shaped carti- 

 lages, one of which surrounds the eyeball on each side, embedded 

 in its sclera; they cannot be seen in a dissection. 



The auditory capsules are a pair of large, globular structures 

 which lie at the hinder end of the skull, one on each side of the 

 brain case. Each capsule consists largely of cartilage, and is partly 

 overlapped by the lateral edge of the parietal bone dorsally and by 

 that of the parasphenoid bone ventrally. 



Three cartilage bones are present in each capsule,— the opis- 

 thotic, the prootic, and the operculum. The opisthotic lies at the 

 lateroposterior corner of the cranium and forms the hinder end 

 of the skull at this place. The prootic lies directly in front of, 

 but not in contact with, the opisthotic and forms the forward end 



