'il6 Vertebrata. 



Lacerta) ; in certain other Lacertilians quite transparent- In the- 

 Ascalabotidse, a few other Lizards, and in the S n a k e s , it is 

 not only transparent, but remains drawn over the eye, with its upper 

 edge fixed to the upper eyelid, so that there is an enclosed space in front 

 of the eye ; these animals seem to be unable to " shut " the eye, the 

 transparent lid looking like a cornea, but as a matter of fact it is 

 always closed. There is usually a nictitating membrane. 

 Besides a lachrymal gland, a lachrymal duct and a Harderian gland 

 are also present. For the parietal eye, see p. 337. 



Auditory organ. The cochlea of most Reptilia is as little 

 developed as in Fish and Amphibia, and is only a small evagination ; 

 in the Crocodilia, however, it reaches a much higher stage of devel- 

 opment, as a fairly large, closed tube. The outer wall of the skull, 

 which lies above the cochlea, is pierced in the Reptilia by an opening, 

 covered by connective tissue, the fenestra rotunda ; above the 

 sacculus there is, as in Amphibia, a fenestra ovalis, which is 

 closed by a process of the pro-otic. There is usually a tympanic 

 c av i t y, closed towards the surface by a tympanic membran e,* 

 which lies in a shallow groove, not, as in Amphibia, at the level of 

 the rest of the skin. The tympanic cavity in the Lacertilia, as in the 

 Amphibia, communicates directly with the mouth by a wide aperture ; 

 in the Chelonia and Crocodilia, on the other hand, by a narrow 

 canal, the Eustachian tube. The Crocodiles are peculiar in 

 that the tympanic cavity is connected with air spaces in the 

 wall of the skull, and that the two Eustachian canals open by a 

 common aperture into the mouth, not far behind the internal nares. 

 In the Ophidia and some Lacertilia tympanic cavity and membrane 

 are entirely absent. There is a columella, like that of the 

 Amphibia ; its flattened portion fits into the fenestra ovalis, and 

 the other end is attached to the tympanum, when there is one. An 

 "external ear" occurs, in the Crocodilia, as a flap or fold of 

 the skin covering the drum externally. 



Teeth occur, in most Reptiles, on premaxilla, maxilla, and 

 mandible ; in Snakes (in which the premaxilla is usually edentulous) 

 and in Lizards, they are often present on the palatine and pterygoid 

 also, whilst they are entirely absent from the Chelonia. They are 

 usually attached to the bdines by osseous tissue; only in the Crocodilia 

 are they implanted in sockets. Replacement teeth are formed con- 

 tinuously throughout lifej the old ones fall out; the osseous 

 substance which attached them to the bones and the lower portions 

 of the teeth themselves, being reabsorbed. The teeth are usually 

 simple, most often conical; sometimes distally compressed and 

 pointed ; sometimes they are knob-like. Usually, all the teeth in one 



* A tympanic cavity is present in Chameleons, and. is closed externally by an 

 undifferentiated portion of skin. A specially developed tympanum is wanting. 



