Tin-: NAUTILUS. 41 



Polygyra and Triodopsis ; Mullanl, appressa, etc., form passages from 

 Triodopsis to Mesodon ; and through germana with its allies on either 

 side, Mesodon flows into Stenotrema. The recognition of the fact 

 that these sections are all varying manifestations of one type, and 

 that a native American one, lead me to associate them under the 

 oldest name, Polygyra, in my check-list of our land shells.' 



So much for the one side. And on this side there is full as much 

 danger in holding extreme views, as on the side of excessive analysis. 

 Let us not profess sweeping views on coalescence of minor groups 

 until we have the species which actually show transition ; and (to 

 pass from generalizations to a special case), it may be noted here 

 that while the species of the Polygyra-\- Triodopsis-\- Me^odon-\- Steno- 

 trema group, invariably have a reflected lip, the two species belong- 

 ing (as I claim) to Polygyrella have a blunt lip, not in the least ex- 

 panded or reflexed. I prefer to keep very different things apart. 



Dr. Cooper's original description is excellent, but the figures are 

 bad. The latter are copied by Tryon in the Manual of Conchology. 

 Binney described a wholly different shell in his two publications — 

 a shell which has, he states, an expanded lip. In his Manual of 

 American Land Shells the species is said by him to have four whorls 

 and is placed in Polygyra. Still later (3rd Supplement to Terr. 

 Moll. V.) Mr. Binney seems to entertain a suspicion that the Trio- 

 dopsis Rojperi Pils. (which he places in Polygyra f) is the same spe- 

 cies. An examination of the type would doubtless have convinced 

 Mr. Binney that it is, as Dr. Cooper states in his original descrip- 

 tion, most intimately allied to Polygyrella polygyrella. Figure 81 

 of the Manual of American Land Shells is incorrect in showing 

 the parietal tooth too far within tlie aperture. The original figures 

 have the same defect. 



With Polygyrella polygyrella, this species agrees in general form, 

 color, sculpture and texture, as well as in the form of the aperture 

 and the blunt, not at all expanded lip. In texture and character of 

 the lip, both species are very different from Polygyra and Triodopsis; 

 the species of these last two sections having the lip expanded and re- 

 flexed. 



The section Polygxjrella may be defined thus : 



Shell disk-shaped, the spire nearly flat, periphery rounded, even 

 in the young ; umbilicus wide within, showing all the whorls. 

 Texture somewhat vitreous and subtranslucent ; ribbed-striate above, 



1 Proc. Acad. Nat. Sci. Phila., 18S9, p. 193. 



