PHYLOGENIC STAGE OF THE GASTROPODA 473 



brian, where others suddenly appear, it is necessary, in the 

 interest of fairness, to admit that the other Upper Cambrian 

 genera may have existed in earlier time. This admission breaks 

 the force of argument which otherwise would accompany the 

 comparison of Lower and Upper Cambrian faunas, as a develop- 

 mental sequence. Yet comparison of the Cambrian fauna with 

 the Ordovician Gastropoda, which are numerous as fossils, is 

 competent to decide whether the Upper Cambrian known types 

 are a fair representation of the marine fauna of that time — 

 which, indeed, they appear to be — and this fauna indicates 

 that the Lower Cambrian known fossils are such as should be 

 expected, provided the group is near its initial, from a long 

 curved conical shell, but that also there may have been many 

 other simple-structured species. 



IMMIGRATION THEORY. 



The Baraboo, Wis., fauna is one which contains several gas- 

 tropod genera whose geologic and biologic relations are, I think, 

 very characteristically Cambrian, and which are of prime impor- 

 tance. This fauna has been described by R. P. Whitfield,' and 

 was referred by him to the Lower Magnesian limestone, an 

 equivalent of the New York Calciferous sandrock. It is from an 

 older formation, as indeed suspected from the first, and is in the 

 Potsdam equivalent. C. P. Berkey has found the fauna at Tay- 

 lor's Falls, Minn., in the Dresbach formation,^ in the zone of Lin- 

 gulepis piimiformis Owen, with Litigula ampla Owen, Obolella polita 

 Hall and Hyolithes primordialis Hall. These gastropod species, 

 which were first discovered around the Cambrian Islands of the 

 Baraboo region, as well known, recur at the Cambrian shore 

 region about Taylor's Falls, Minn., in the basal conglomerate, 

 and less frequently in the adjacent brachiopod shales. Any con- 

 siderable geographic distribution of them in the then seaward 

 direction seems to be disproved because, although the Dresbach 

 formation is soon concealed, it emerges at a greater distance 

 around Trempealeau and La Crosse, Wis., and Dresbach, Minn. ; 



' Geology of Wisconsin, Vol. IV., pp. 194-203. 



"" American Geologist, Vol. XXI (1898). pp. 270-94. 



