474 FREDERICK W. SARD E SON 



and there the species of gastropods do not reappear, excepting 

 the Hyolithes primordialis Hall. There is, therefore, a good 

 reason for considering the Baraboo gastropod fauna as one 

 occurring only near the Cambrian land, excepting the Hyolithes, 

 which occur also in the normal marine sediments. 



This is, I think, the aspect of the known earlier Cambrian 

 moUuscan faunas, that Hyolithes and other supposed pteropodous 

 similar genera are common and widely distributed in the marine 

 sediments, and gastropods, Scenella, et al., are rare, local, and 

 probably lived adjacent to land. Their occurrence may, there- 

 fore, require an interpretation similar to that which T. C. Cham- 

 berlin^ makes for the fish of the Paleozoic, which he shows to 

 have probably originated in the rivers and later become marine 

 inhabitants, for geologic reasons becoming then abundant fossils. 

 The Gastropoda may be immigrants to the marine habitat in 

 Cambrian time. Assuming that there was a fresh-water molluscan 

 fauna in Cambrian and Ordovician times, and that it is unknown 

 to us, except as it is reflected in the early immigrants and in the 

 preserved land and fresh-water faunas of later ages (Devonian to 

 Recent), then we are able to explain some peculiarities of the 

 Cambrian Gastropoda. From an ancestral gastropod stock in 

 fresh water several immigrations to the sea may have taken place 

 in Cambrian to Ordovician time, and that is what causes the 

 emergence of new genera which we see in the marine fossils. 



To illustrate from the region of the upper Mississippi valley, 

 where the encroachment of the Cambrian Sea, as shown by Wal- 

 cott, was in later Cambrian time, the oldest exposed strata are 

 in the Dicellocephalus zone and of the Potsdam equivalent, and 

 above this follows the Magnesian series, which is the Calciferous 

 sand rock equivalent. Both have a known gastropod fauna; 

 though it is necessary to offer a correction here, since inadvert- 

 ently lists of fossils, such as that in North Atnerican Geology and 

 Palceontology by S. A. Miller, refer species of the upper division 

 only to the lower or Potsdam, and those of the "Potsdam" all 

 to the upper division, which is the Calciferous or "Lower Mag- 

 nesian," excepting Hyolithes primordialis Hall, which is rightly 



'Jour. Geol., Vol. VIII, pp. 400-412. 



