476 FREDERICK W. SARDESON 



glomerate, breccia, oolite, or mud balls. Rounded pebbles one 

 or two inches in diameter occur even in the St. Lawrence forma- 

 tion of Red Wing, Minn. The species of fossils are quite all 

 different from those of eastern America. Also the St. Peter 

 sandstone (Chazy) has its peculiar molluscan species, but follow- 

 ing this formation the Galena (Trenton) has numerous species 

 which are largely identical with those from other regions, and 

 surely of normal marine habitat. 



Thus in this region one appears to see the Gastropoda crawl- 

 ing off the rocks of the Cambrian seashore, migrating seaward, 

 and becoming in Ordovician time the companions everywhere of 

 the Brachiopoda and Trilobita, and this has suggested to me the 

 probability of non-marine origin of MoUusca. 



Such a theory does not change in any way the estimated 

 imperfection of the geologic record as to the Gastropoda, except 

 that the meaning of the same is differently taken when the 

 ancestral stock is assumed to be the fresh-water fauna and not 

 the salt-water one. The fresh-water and land faunas are totally 

 unknown, and our Cambrian gastropods are immigrants, and 

 thence the marine ancestors of Prosobranchia. The later emerg- 

 ing groups of Pulmonata and Ophistobranchia, for example, are 

 not necessarily descended from the known Eo -Paleozoic, but 

 would be descended from that unknown common stock. Cepha- 

 lopoda may be derived from the first immigrated, widely found 

 marine pteropodous genera Hyolithes, Hyolithella, etc. The 

 class Pelecypoda, all but unknown in the Cambrian, might be 

 taken as the third immigration seaward, and Scaphopoda the 

 fourth. In this connection, I recall a former occasion for sur- 

 prise when, in discovering Ordovician fossils in the St. Peter 

 sandstone, I found that the Pelecypoda were both present and 

 predominated, and Gastropoda and Cephalopoda were also in it. 

 That formation is as unfavorable for fossils as any of the Cam- 

 brian fossil-bearing zones well could be, and therefore the Pele- 

 cypoda seem to have just arrived. 



THE EARLIER CAMBRIAN FAUNAS. 



A review of the Cambrian representatives in the light of these 

 theories may elucidate somewhat the phylogenic stage, whether 



