480 



MESOZOIC ERA— AGE OF REPTILES. 



Two very strange and characteristic groups of bivalve-shells occur 

 here, and are very abundant, viz., the Rudistes or Hippuritidce and the 



Chamidce. In the former, one valve is small 

 and often flat, while the other is enormously 

 elongated like a cow's horn. In the latter 

 one or both valves are elongated, and often 

 coiled in the manner of a ram's horn. We 

 give some figures (789-792) from foreign 

 localities and some (787, 788) from our oivn 

 country. 



Among Gasteropoda (Figs. 793-795), the 

 beaked or siphonated kinds are now for the 

 first time abundant, as in the present seas. 



Fig. 791. 



Fig. 790. Fig. 792. 



Figs. 789-792.-789. Hippurites Toucasiana, a large individual with two small ones attached (after 

 D'Orbigny). 790. Section of a Radiolites cylindriasus, showing structure. 791. Upper Valve of 

 Eadiolites mammelaris. 792. Caprina adversa (after Woodward). 



Among Ceplialopods the Ammonites and Belemnites still continue 

 in great numbers and size, but they die out at the end of this period 

 forever. In the Cretaceous of the Western Plains some Ammonites 

 have been found over three feet in diameter (Dana). This family 

 seemed to have reached its culmination just before its extinction. But 

 what is still more remarkable is the introduction of many new genera 

 of very strange and unexpected forms. These are sometimes partly 

 uncoiled, as in Scapliites (boat), Crioceras (ram's-horn), Toxoceras 

 (bow-horn), Ancyloceras (hook-horn), Hamites (hook) ; sometimes 

 completely uncoiled, as in Baculites (walking-stick) ; sometimes coiled 



