126 BULIMULUS. 



stride, in the vast majority of species; but in a few there are spirals, 

 without distinct grated pattern. No BULIMULUS has this type of 

 apical sculpture. 



It is, therefore, possible to ascertain the generic position of any 

 specimen in which the apex is unworn, from the shell alone, if a 

 lens of high power be used in 'the examination. The exceptions to 

 this statement being extremely few in number, so far as my obser- 

 vations extend. On the other hand, it is frequently impossible to 

 decide with certainty the position of species which one may know 

 only by descriptions and figures, since these do not, in most cases, 

 afford the requisite information upon the minute sculpture of the 

 apices. In the grouping of such forms in this work, the general 

 features of the shell have served as a guide, but it must be expected 

 that further examination of specimens will result in a certain num- 

 ber of transpositions and exchanges of species between the genera 

 Bulimulus and Drymceus. 



Bulimulus differs from Plekocheilus and Auris mainly in the 

 simpler shell, without the modifications of lip and columella char- 

 acteristic of those genera. 



CLASSIFICATION. Bulimulus has been subdivided into a large 

 number of subordinate groups, based upon the contour and texture 

 of the shell. The subgenera so formed are in many cases neither 

 consistent geographic groups, nor are the characters of shell-contour 

 upon which they are grounded, correlated with anatomical distinct- 

 ions. It is here proposed to abandon the divisions founded upon 

 these characters, and to substitute a classification based primarily 

 upon the sculpture of the nepionic or apical whorls. This feature is 

 in most species readily observable in adult specimens with a lens of 

 ordinary power; and the examination of extensive series shows it to 

 be remarkably and to an unexpected degree constant throughout 

 large groups of species. The groups indicated by the several types 

 of embryo sculpture are confirmed by the anatomical characters, so 

 far as these are yet known, and they are in every case which has 

 come under my observation, consistent geographically. 



In course of the development in this volume and the one to follow, 

 of the system outlined below, it will be observed that in many cases 

 a series of species " parallel " in contour, texture and general aspect 

 recurs in each of the main divisions of the genus. Thus the groups 

 Peronceus (Chili, Peru), Pleuropyrgus (Galapagos) and Plicolumna 

 (Lower California) are very similar in their narrow, long contour, 



