504 LACERTILIA chap. 



or wedge-shaped. Dorsally it is continued as fibro-cartilage, and 

 the whole ring acts as an articular pad instead of the joint. 

 Chevron-bones are common in tlie tail. 



The ribs are bifurcated, but the tubercular portion is frequently 

 reduced. Tlie post-thoracic ribs are usually very slender, and 

 so long that they meet each other in the middle line, in 

 this case bearing an extraordinary resemblance to the so-called 

 " abdominal ribs " of other reptiles. 



The bony frame of the skull is slender. There is a complete 

 absence of l)ony arches spanning over the temporal fossae, or 

 bordering the orbit, wiiich is open posteriorly. The upper jaw, 

 owing to the slender and flexible nature of the respective bones, 

 is movable upon the rest of the skull ; in tliis respect not unlike 

 the upper jaw of a duck. The dentition is pleurodont and the teeth 

 are minute. The eyes of the typical Geckos are peculiar. They are 

 covered with an absolutely transparent skin, shaped like a 

 watch-glass, beneath which the eye moves freely, while the true 

 upper and lower lids are reduced to tiny folds. The covering 

 " watch-glass " is probably a modification of the nictitating mem- 

 brane. In the Eu1)lephariuae, however, and in the few species 

 of the Geckonine genera Aelurosaurus of Borneo and Australia, 

 and Ptenopus of South Africa, the upper and lower lids are 

 present and movable. The pupil contracts mostly into a vertical 

 slit, except in the few diurnal kinds, e.g. Phehuma, of the islands 

 in the Indian Ocean, and the African Lygodactylus. 



Another peculiarity of at least many Geckos is the extra- 



1 ordinary development of tlie endo-lymphatic sacs of the ear, 



wliich, beins filled with the chalk -like otoconia or otolithic 



crystals, perforate the skull, and are stowed away in the shape of 



a pair of large bags behind the ears, or on the sides of the neck. 



The skin exhibits considera])le variety. It is mostly soft 

 above, with little granular tul)ercles, sometimes containing small 

 dermal ossifications or calcifications. Tlie latter are most de- 

 vel()])ed on the head, where they occasionally fuse with the 

 underlying hones. A few species of Tarentola possess supra- 

 orbital bones, independent remnants of such osteoderms. The 

 ventral surface is generally covered with small imbricating 

 scales, but in some genera, e.g. Homop)holis, such scales occur also 

 on the dorsal surface, reaching their highest development in 

 Teratoscincus (p. 507). In n fow forms, notably in Pfychozoon 



