12 REPTILES AND BATRACHIANS 



Regeneration of lost parts does not take place, although, 

 as Gadow has shown, the injuries to the shell are made 

 good by new growth of bony and horny tissue, after the 

 necrotic portion has been cast off. 



A tail is always present, but differs much both in length 

 and structure, according to genera and species, this organ 

 being sometimes covered with horny or bony tubercles, 

 while in many the tip is provided with a sharp, nail-Hke 

 spur. The toothless jaws are covered with cutting horny 

 sheaths, which may be serrated and constitute false teeth. 

 The neck, which varies much in length, may be either 

 completely or only partly withdrawn into the shell, in 

 some forms simply sideways (Pleurodira) , or by a sigmoid 

 curve in a vertical plane (Cryptodira). The eye is rather 

 small and protected by an upper and a lower lid, and a 

 transparent membrane (the nictitating membrane), which 

 moves horizontally ; the pupil is always round. The sight 

 and senses of taste and touch are well developed, that 

 of hearing, however, is very imperfect, especially among 

 aquatic forms, some of which are devoid of an exposed 

 tympanum or ear-drum. 



All tortoises, terrapins, and turtles are oviparous, laying 

 eggs which may be round, oval, or elliptical, and as a rule 

 hard-shelled : marine turtles, however, produce eggs, the 

 shell of which is imperfectly calcified : they are invariably 

 buried in the ground and hatched by the heat of the sun. 

 Hibernation takes place in the temperate zones, the period 

 varying in length according to the chmate. Aquatic 

 species do so generally at the bottom of the ponds or 

 rivers they live in, or in the mud on the banks, while the 

 land forms secrete themselves in the earth and sand, a 



