150 



MARINE DEPOSITS. 



of different animals now living in the seas. Polypa'ria. more or 

 lest) analogous to those which form coral reefs (Jigs. 220 to 223- 

 p. 141), are highly characteristic; encri'nites (Jigs. 235 to 237), 



Fig. 238. Cida'ris corona'ta. Fig. 239. Different joints of Encri'nites. 



or the remains of their joints (fig. 239) the echini 'deas (figs. 

 338 to 241). Not one of these organic todies is found in fresh 

 water 



Fig 240. financhytes ovatus 

 (from the Parisian chalk}. 



Fig. 24l.Spata'vg-us ambula 

 (from the chalk of the Pyrenees). 



Among the marine univalves there are 

 some which are more or less analogous to 

 those of fresh, water, mentioned (p. 148), 

 although they are thicker, and more gene- 

 rally covered with tubercles (Jig. 242). 

 But, setting aside those on which at first 

 sight there might be some doubt, there are 

 many others which are sufficiently charac- 

 teristic : these are shells whose aperture is 

 terminated by a canal of greater or less 

 length, and belong either to the genus ceri'- 

 thium (fig. 243), of which a small number 

 of species lives in fresh water, or to \he 



genera mu'rex (fig. 244), volu'ta (fig- 245), &c. ; they are all 



marine, and abound in calcareous deposits. 



Fig 242. Turbo costa'rius. 



