ASHFORD : ON BANDED LAND SHELLS. 9 1 



and abnormal form. Why these strife should not accept the 

 coloring matter is not quite clear. Whether by their greater 

 thickness and opacity they do not allow the pigment to be seen 

 through their upper surface, or whether from a want of homogeneity 

 common to all uncrystallized matter in course of concretion, these 

 ridges are more indurated than the rest of the shell structure, and 

 thus less capable of absorbing the pigment, can be determined 

 only by a microscopic examination of a carefully prepared section. 

 As these ridges gradually invaded the region of the zones, they 

 broke here and there into the continuity of the bands; peculiarities 

 thus formed were transmitted to broods which displayed still 

 further development of stride, and a change once set up it is not 

 difficult to follow its progress. How far does the existing state 

 of the four species under consideration bear out these conclusions? 



The four species have evidently not proceeded pari passu in 

 their march toward the disintegration of the colored bands. The 

 modification is most advanced in the case of B. acuins (among 

 the individuals of which species it is very rare to find one with 

 perfectly continuous bands though one of the two may often be 

 so), and is least advanced in the case of H. e/iceton/m (among 

 which broken bands are the exception). H. caperaia and virgata 

 take their places between these two extremes in the order named. 

 Now when we come to examine the sculpture of these species we 

 find, if we direct our attention to large numbers of individuals 

 and not to exceptional cases, that B. aciitus has the ridges most 

 pronounced and irregular, while H. ericetoriim has a comparatively 

 smooth shell. The other two in this respect also, take interme- 

 diate positions and in the same order as before. So far then as 

 regards these species, their several tendencies to a disintegration 

 of the bands are in direct proportion to their tendencies to 

 acquire prominent, irregular, enamel-like ribs. 



The interruption of the upper zone having once commenced 

 it may be observed to have taken two courses in the progress of 



