492 FREDERICK W. SARD ESQ N 



The third immigration is that represented by the Baraboo 

 fauna, with curved conical shells, retrorse shells, and sinistrally 

 spiral ones. In the Calciferous there are the shorter cones, 

 Tryblidium, and the sinistral spiral Maclurea. This group is 

 somewhat problematical, being doubtfully Prosobranch and 

 possibly Ophistobranch, recognizable species of Order II, Ophis- 

 tobranchia, appearing first in the Devonian and Carboniferous, in 

 the genus Actaeonina. Order III, Pulmonata, at the same time 

 appears in the genus Archseozonites and Dendropupa. Those 

 three genera are primitive-looking, but of no recognized exact 

 relation to any contemporaneous or preceding ones of the Proso- 

 branchia among fossils ; and it may be considered that their 

 ancestors of Cambrian time were in the fresh-water fauna, from 

 which also the Prosobranchia separated by migrating seaward in 

 Cambrian time. The practically sudden emergence of marine 

 Pelecypoda in the Ordovician time finally permits a theory of 

 their freshwater origin, at that time, in accord with the view 

 that Mollusca as a whole began in fresh water habitat. 



Frederick W, Sardeson. 

 Minneapolis, Minn. 

 May 30, 1903. 



