582 REPTJLIA. 



from their close approximation to Fishes, have neither tympanum 

 nor columnella ; and thus, like Fishes, can only hear in an aquatic 

 medium. 



(639.) The membranous labyrinth of Reptiles (jig> 264, #, &, c) 

 corresponds in its general conformation with that of Fishes, pre- 

 senting the same semicircular canals, ampullse,and vestibular cavity ; 



Fig. 264. 



and moreover, the sacculus contains cretaceous concretions, or oto- 

 lithes of a similar character. But in this class the membranous 

 canals become enclosed in a bony sheath, moulded as it were upon 

 their outer surface ; which is another very important step towards 

 perfecting the auditory apparatus. 



(640.) Neither must we omit to mention, that in the highest 

 of the Reptilia, as for example in the Crocodile, the first rudiment 

 of a cochlea makes its appearance, although as yet in a form of 

 extreme simplicity. This portion of the organ of hearing, which, 

 from the elaborate structure that it presents in the higher Verte- 

 brata, must be regarded as being importantly connected with cor- 

 rect audition, is seen in this, the earliest stage of its developement, 

 to be a simple conical appendage to the sac of the vestibule ; and, 

 on opening it, it is found to be divided by a central cartilaginous 

 septum into two compartments, which are however continuous 

 with each other at the apex of the cone. One of these com- 

 partments or canals opens at one extremity into the vestibule, 

 while the other communicates with the tympanic cavity by a 

 very small aperture closed with a thin membrane. Thus, there- 

 fore, although the entire organ resembles a simple canal bent 



