818 THE PALEONTOLOGY OF MINNESOTA. 



[Classification. 



acceptable subdivision of the class would result if the various groups were reduced 

 in rank, the subclasses to orders, the orders to suborders, and the suborders to 

 superfamilies. The divisions a and b would in that case take the rank of subclasses. 

 Though inclined to favor such a proceeding, we refrained from carrying it out 

 because it entailed more responsibility than we cared to assume at the present 

 time. 



Class GASTROPODA, Cuvier. 



Division a. 

 I. Subclass SCAPHOPODA, Bronn. 



Mollusks having neither eyes nor a distinct head, yet with the lingual armature. 

 Sexes separate. Foot vermiform, lobate. Shell a hollow cylinder, open at both 

 ends. Dentalium is the principal representative. 



II. Subclass POLYPLACOPHORA, Blainville. 



Symmetrical mollusks having a distinct head, but neither eyes nor tentacles. 

 Foot broad. Shell consisting of eight moveable calcareous plates arranged in a 

 row. This division includes the chitons. 



III. Subclass PTEROPODA, Cuvier. 



Head and organs of sense rudimentary. Foot modified into two lateral wing- 

 like expansions used in swimming. Animal naked er protected by a shell. 

 Order Thecosomata, Blainville; with mantle and external shell. 

 Order Gymnosomata, Blainville; both shell and mantle wanting. 



IV. Subclass DOCOGLOSSA. (Provisional.) 



Approximately symmetrical gastropods, provided with dish-shaped or conical 

 shells, or with spiral shells coiled in the same plane. A very primitive group of 

 which the Patellacea and Bellerophontacea, comprised in the provisional order 

 Proteobranchia, constitute the bulk, if not the whole. 



V. Subclass OPISTHOBRANCHIATA, M. Edwards. 



Branchiae more or less free, behind the heart. Animals rather highly developed, 

 naked or with a shell; the sexes united in the same individual. 

 Order Nudibranchiata; without a shell. 

 Order Tedibranchiata ; with a shell. 



