228 Notes on the Osteology of the Shad, 



the dorsal aorta in front of the anterior extremity of the air 

 bhxdder, with which it is connected/ 



The cartilaginous processes which pass out from the cartilage 

 plate on the cranial floor, are occasionalh' continuous with it, 

 but in most cases they articulate with it, and with each other 

 in some instances. Some of the process are double, others are 

 rudimentary. When removed from the bones in which they are 

 imbedded, they present, with some modifications, the form of 

 the membraneous labyrinth of other fish, and in consequence, I 

 have designated them, with the membraneous sac attached to 

 the cartilage plate as the posterior auditory apparatus. By care- 

 ful examination, I am satisfied that the cartilage plate and its^ 

 processes represent neural arches and epicentrals, and that the 

 crowding together of the anterior body segments has resulted in 

 a coalescence of the anterior vertebral elements with the pos- 

 terior cranial bones, and a consequent modification of the orig- 

 inal auditory apparatus and the formation of a secondary one on- 

 the primitive type. 



The absorption or non-development of cartilage in the pro- 

 cesses above described would give us a membraneous labyrinth 

 of the usual type, and the manner in which such a structure 

 could be formed is thus strongly indicated. Such an origin 

 would give good reason for regarding the auditory apparatus of 

 the Cyclostomata as the most primitive known among verte- 

 brates. 



' The connnection between the air bladder and the auditory apparatus 

 seems to be very primitive in the Shad, and is very interesting, The mod- 

 ification of the auditory apparatus will be dealt with in greater detail in 

 connection with a description of the nervous system now in preparation. 



