166 



SHELLS AND SHELL-PISH. 



more than usually produced, resemble Bulimi, but with 

 this difference, that the whorls are more ventricose 

 and the top of the spire more obtuse; in some the 

 base of the pillar forms a small sinus or notch before 

 it joins the thickened outer lip, analogous to the Acha^ 

 tince ; but in that the margin is always thin, and 

 the spire more contracted and pointed : something of this 

 form belongs to our next sub-genus Geomitra {fig. 20.), 

 founded upon a most extraordi- 

 nary little shell discovered by 

 Mr. Lowe in Madeira, and exhi- 

 biting the only instance of coro- 

 nated nodules on the whorls we are acquainted with in 

 this family. The form is that of the last type ; but the 

 mouth is thin and completely circular, like that of a 

 Pupa, to which genus we consider it 

 leads. As connecting Geomit7'a with He- 

 mitrochus, we here place the fifth sub- 

 genus Gonidomus {fig. 21,), which, in 

 the distortion of its whorls, is a com- 

 plete prototype of Strepsaxis. The ge- 

 nera Helix and Geotrochus would thus 

 appear to form two circles, whose sub- 

 ordinate types may be thus stated. 



Analogies of the Genera Helix and Geotrochus. 



Analogical Characters. 



Sub-genera of 

 Helix. 



Helix. 

 Zonites. 



Epistylia. 



Strepsaxis. 

 Hemicycla. 



fBody.whorl large, ventricose ; 7 

 X umbilicus none. j 



f Body-whorl depressed, narrow ; ) 

 \ umbilicus open. j 



f Spire elevated, obtuse ; body. 1 

 \ whorl depressed, the margins > 

 L convex. j 



Spire and whorls distorted. 



C Aperture or outer lip perfectly \ 

 I round. \ 



Sub-genera of 

 Geotrochus. 



Geotrochus. 

 Hemitrochus. 



Gonidomus. 

 Geomitra. 



These sub-genera agree sufficiently well to render any 

 additional illustration unnecessary. We shall therefore 



