MES0201C ERA. AGE Off REPTILES. 



353 



FIG. 312. Hippurites Toucasiana, a large individual with two small ones attached. 

 (After D'Orbigny.) 



Echinoderms are now almost wholly of free forms. 

 The highest echinoids are especially abundant. And, 

 what is remarkable, 

 those from the chalk 

 are very like those 

 still living in deep 

 seas. The reason of 

 this is that deep-sea 

 conditions, and there- 

 fore species, change 

 far more slowly than 

 those of shallow water 

 and land. 



Bivalve Shells 

 Among the immense 

 number of bivalve 

 species found here, 

 we mention only 

 the oyster family, of 

 which there are many 

 species, and the 



FIG. 315. FIG. 316. 



FIGS. 313-316. 313. Toxoceras annulare. 314. 

 Hamites attenuatus. 315. Ancyloceras spinige- 

 rum. 316. Baculites anceps, x $. (After Wood- 

 ward.) 



strange Hippurite 



family (Fig. 312). Surely no one, from its general" form, 



would imagine that these latter were bivalves. 



Cephalopods. The Ammonites and Belemnites still 



LE CONTE, GEOL. 23 



