S4S ANATOMY OF VERTEBRATES. 



three semicircular canals, and otolithic sacculus containing a semi- 

 fluid cretaceous substance : it is proportionally smallest in Pipa. 



All serpents have the internal organ of hearing, similar in the 

 main to the above : the base of the stapes closing the foramen 

 vestibuli, is connected, in most serpents, by a long slender bony 

 style, fig. 97, 16, through the medium of a cartilage and liga- 

 mentous fibres to the skin, which shows no sign of ear-drum or 

 external meatus. No air is admitted by an eustachian canal to 

 the cellular substance traversed by the tympanic ossicle ; and of 

 this there is no trace in Tythlops and Rhinophis. 



In Lacertians, the modification of part of the integument for 

 the special reception of sonorous vibrations is resumed. In the 

 Iguana, the ear-drum is partially protected by its oblique position 

 and by a rising or fold of the skin at the back part of its circum- 

 ference. It consists of the proper fibrous tissue of the tympanic 

 membrane, covered externally by a thin layer of epithelium, and 

 internally by the lining membrane of the tympanic cavity. The 

 communication between the membrana vestibuli and membrana 

 tympani is by the stapedial disc, the columelliform ossicle and the 

 terminal or ' malleal ' cartilage. The eustachian canal is rela- 

 tively narrower, and its course more oblique than in the Frog : the 

 otolite is a lenticular calcareous body, firmer than in the Frog. 



In the Chelonia the ear-drum is again masked by unmodified 

 integument ; in the Turtle ( Chelone mydas) it is covered by the 

 second scale counting upward from the articulation of the lower 

 jaw. The membrana tympani is, however, distinctly formed, 

 thicker and more opake than in Batrachia or Lacertia, and con- 

 vex outwardly. The long columelliform ossicle fig. 92, ie', is 

 connected with it by a discoid cartilage, and, at the opposite end, 

 penetrates and closes the vestibular orifice by a sub-cartilaginous 

 plate. The tympanic cavity is divided into two parts by a bony 

 septum, intercommunicating at the columellar canal : the inner 

 or antevestibular part also communicates with cells on the mastoid. 

 The eustachian canal is narrow, and descends behind the arti- 

 culation of the mandible ; its palatal opening is more remote from 

 that of the opposite ear than in other Reptilia. The proper cap- 

 sule of the labyrinth is cartilaginous : besides the three semi- 

 circular canals and the otolithic capsule, there now buds from the 

 vestibule a beginning of a cochlea, with a corresponding small 

 opening into the tympanic cavity. 



These approximations to the higher structure of the internal 

 ear are more conspicuous in Crocodilia. The cochlear process is 

 conical, with the apex, fig. 230, a, slightly bent; its cavity is 



