THE RECORD FROM FOSSILS. 



105 



In general the Paleozoic (2-4) lamellibranchs were commonly without 

 siphons, although some of the siphonate type even then existed ; the 

 modern forms, on the other hand, are siphonate as a rule, though 

 many of the asiphonate forms still exist. The most remarkable 

 point in the history of this class was the development of a very pecu- 

 liar order in the Cretaceous (9). This order (Rugosites) is very 



FIG. ii. Diagram illustrating the history of the Mollusca. C Cephalopoda. 

 P Pteropoda. H Heteropoda. G Gasteropoda. L Lamellibranchiata. 



unlike any other mollusks, and it suddenly appeared without warning 

 in the Cretaceous and disappeared at its close. 



Gasteropoda (snails). The history of this class is quite parallel to 

 that of the last. Beginning with the Silurian (2), they have been at 

 all times very abundant. With the Mesozoic (5-8), the older forms 



