150 Notes on certain Terrestrial Mollusks. • 



the Terrestrial Mollusks haying two tentacles with basal 

 eyes. 



Pfeiffer, in his Monographia, and Gray in the British Museum 

 Catalogue, place all such Mollusca in the Order Pneumonopoma, 

 which they divide into the two sub-orders, Opisophihalma and 

 fictophihalma (with reference to the position of the eyes behind 

 or in front of the tentacles), the former Order including the Fa- 

 mily Acicalacea, and the latter Cyclostomacea and Helicinacea \ , 

 one of the characters of the Order, and of course of the sub- 

 orders and families, being an operculum. These authors, more- 

 over, in the further subdivisions into genera, rely greatly, and 

 in my opinion unduly, on the nature of the operculum. 



The arrangement thus briefly described, must therefore be 

 remodelled, since it now improperly excludes Proserpinacea, 

 which family, though inoperculate, is from all its other charac- 

 ters, both of animal and shell, entitled to admission into the same 

 Order which contains the sister family Helicinacea. 



III.— On - the Structure of the Axis of the Shell of 

 Cylindrella. 



On a late examination of some of the Jamaica CyUndrellce, 

 I noticed the curious structure of the axis of C. elatior C. B. 

 Ad., and was led to compare it with other species. Looking at 

 these shells externally, it has probably been assumed that the 

 axis is perpendicular, supporting the revolving septa, in the 

 same manner as a column forms the central support of a spiral 

 stairway. This is not, however, universally the case. I was 

 surprised to find that the axis in C. elatior (PI. v. fig. 19) is 

 spiral, the diameter of the volutions increasing gradually to- 

 wards the base of the shell, and to such an extent as to exhibit, 

 looking into the aperture, an open perforation, equal to about 

 one-third of the diameter of the shell ; the lower whorls being 

 like a spiral stairway constructed with a conical well- hole, 

 instead of a column. 



The formation of the axis in C. tenera C. B. Ad., and 



