4 Notes on certain Terrestrial Mollusks. 



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 Prose rpinacea^ 

 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.— O.v THE Structure of the Axis of the Shell of 

 Cylinlrella. ,;; 



On a late examination of some of the Jamaica Cylindrelloa^ 

 I noticed the curious structure of the axis of C. elatiov 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 0. elatior (PL 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 

 C. tenella C. B. Ad., is the same, and also, I imagine, in the 

 other Jamaica allied species, although the fact is not discoverable 

 on examination of their apertures. 



The structure of G. Agnesiana C. B. Ad., is shown in PL v. 

 fig. 16. In other species the axis has revolving lamellae, — there 

 are two in C. pruinosa Mor. (PL v. fig. 17), and from three to 

 six in C. Oviedoiana Orb., within each of the whorls, — the num- 

 ber increasing towards the base of the shell. 



