Notes on the Osteology of the Shad. 227 



membrane bone^ lying between the heads of the hyomandibular, 

 contains a canal which communicates with a foramen lying 

 partly in thesphenotic, and partly in the pterotic and opisthotic. 



This foramen opens into a large chamber lying in the prootic, 

 thesphenotic, pterotic, and opisthotic, and opening below into 

 the brain cavity. It contains a large membraneous sac, which 

 receives a branch from the eighth nerve. In the pterotic, just 

 beneath the post-temporal is a large foramen opening into this 

 chamber, and communicating also with a canal in the membrane 

 bones of the pectoral arch. The prootics are co-ossified in the me- 

 dian line and form the anterior part of the floor of the cranium. 

 In the centre of each bone is a large osseous capsule containing a 

 membraneous sac, which receives a branch from the eighth 

 nerve, the branch passing through an oblong slit in the capsule. 

 This slit is the only opening that I could find communicating 

 wnth the interior of the capsule. This capsule and the chamber 

 described above, with their contents, I shall designate as the 

 anterior auditory ap})aratus. 



Covering the capsule nearly, if not entirely, is a plate of cart- 

 ilage which extends backward on the floor of the cranium to the 

 occipital foramen. It is jierforated for the passage of the cran- 

 ial nerves, and gives off, processes in the shape of cartilaginous 

 rods, which pass into the bones of the ear capsule with the ex- 

 ception of the prootic and sphenotic, with which they have no 

 connection. Similar processes completely surround the occipital 

 foramen. 



To the base of the cartilage plate on each side is attached a 

 membraneous sac, containing an otolith. The sac is lodged in 

 a cavity in the basioccipital. The external lateral wall of the 

 cavity is, in most of the specimens, of membrane only, and close- 

 ly applied to it, on the outer side, is a rib-like rod of cartilage 

 which passes downward and backward, meeting its fellow below 



' This bone probably represents the squamosal of higher types. The 

 membrane bones of the pectoral arch in the Teleosts, and probably in all 

 fish, may be derived from lateral line scales. The opercular apparatus 

 is probably derived from similar scales. 



