196 



REPTILIA. 



rally to be fossils in marine deposits ; and tlie preservation 

 of all tlie Chelonians alike is rendered easy by the inde- 

 structible nature of the case in which their bodies are 

 enclosed. 



The Chelonians may be di^dded into sections according as 

 the limbs are natatory, are adapted for an amphibious life, 

 or are fitted for terrestrial progression. In the first of these 

 sections are the true Turtles {Chdoniidw), which frequent the 

 sea, and are distinguished by their depressed and flattened 

 carapace, and by their oar-like limbs. In the second section 

 are the Eiver and Marsh Tortoises, comprising the Soft 

 Tortoises {Trionycidce) and the Terrapins {Emydidcc). In 



the third section are the 

 true Land-tortoises {Tcs- 

 tudinidcc), distinguished 

 by their strongly convex 

 carapace, and limbs ad- 

 apted for walking upon 

 the land. All these tliree 

 sections are represented 

 in past time, the Turtles, 

 Trionycidce, and Emy- 

 didcv appearing for the 

 first time, so far as is 

 certainly known, in the 

 Jurassic series, whilst 

 the Testudinidce do not 

 appear till the com- 

 mencement of the Ter- 

 tiary epoch. Tlie earliest 

 apparent traces of Clielo- 

 nians occur in the Per- 

 mian rocks, in the lower portion, that is, of the New Eed Sand- 

 stone of the older geologists. These traces, however, are not 

 satisfactory, since they consist solely of the footprints of the 

 animal upon the ripple-marked surfaces of the sandstone. 

 Of this nature is the Chelichnus Duncani, described by Sir 

 William Jardine in his classical work on the ' Ichnology ' of 

 Annandale in Dumfriesshire. It is not, however, till we 



Fi". b51.—t'helone Benstedl. Lower Chalk. 



