PHYLOGENIC STAGE OF THE GASTROPODA 491 



tropoda, and the one from which the short conical shells, on 

 the one hand, and the spiral and coiled shells on the other, have 

 descended. Conical shells have arisen also from coiled and 

 spiral ones in later geologic ages. The visceral twist probably 

 correlates with the development of the long asymmetrical cone, 

 or most primitive form, and not with the spiral shell. 



The ancestor of the MoUusca was probably living in fresh 

 water, and there the ancestral gastropods began. The Cambrian 

 Gastropoda which are known to us appear to be immigrants to 

 the sea, the fresh water and land stock remaining unknown to 

 us geologically, though they may be reflected in the known 

 marine immigrants. The Hyolithoidea may represent the first 

 migration seaward. They comprise long conical, more or 

 less curved, often triangular shells, and are the normal marine 

 species, very probably representing the class Pteropoda ; and 

 from the same the Cephalopoda may have derived. 



The second migration comprised the other Lower Cambrian 

 Gastropoda. They are taken to be the ancestral group of Proso- 

 branchia, and to be in process of change from long curved conical 

 to short conical, on the one hand, and, on the other, to spirally 

 coiled. The coiling is at the shell apex first and strongest; i.e., 

 the coiled form of mature shell arises by retardation. The aper- 

 ture is simple and is oblique to the radius of the curve, or, in 

 other words, the shell is not longest on the convex anterior side, 

 for which reason the aperture runs obliquely backward from the 

 inner (posterior) to the outer (anterior) side of the volution. 



The gastropods in the Calciferous formations are readily 

 interpreted as derived from the Lower Cambrian species, with 

 certain exceptions. These are short conical shells, the Doco- 

 glossa. The Rhipidoglossa comprise species of long coils, with 

 more or less changed aperture, which arises essentially by the 

 building forward of the anterior or convex side of the shell, 

 leaving near it a sinus in the margin, e.g., Bellerophon, Pleuroto- 

 maria, and Murchisonia. These forms undoubtedly belong to 

 Suborder i, Diotocardia. To Suborder 2, Monotocardia, belong 

 similarly Platyceras and Subulites, in the latter of which the 

 aperture forms a shallow anterior canal. 



