94 PARTULINA, SECTION BALDWINIA. 



small. Thus the isolation of a few feet seems sufficient to 

 permit recognizable changes both of size and color, though 

 not of form. 



" As these four color-forms occur in the same locality and 

 are only to be distinguished by pattern of the shell, it might 

 naturally be inferred that the embryonic young of the four 

 would be of a common type, perhaps reverting in color to an 

 ancestral form, and in any event not perpetuating the color 

 peculiarities of their respective parents. The contrary is 

 true. The young are not of a common type, but are always 

 distinguishable inter se. Though markedly different in the 

 adult state, there is less difference between the young of the 

 white and the banded form than of the other two, but there 

 is still a recognizable difference. The embryonic young of the 

 pure white form retain the peripheral brown band of the 

 typical horneri until about a third grown when only traces 

 of it are discernible near the aperture and these finally en- 

 tirely disappear leaving the shell pure white. The embryonic 

 shells of the other two forms differ more markedly from each 

 other and from the young of the other forms. 



" The unlikeness of the respective young of several mol- 

 luscan forms would appear to be an excellent criterion of 

 their permanent distinctness, and the writer is inclined to 

 view these two forms as far better entitled to specific rank 

 than many named species of the group whose claim to dis- 

 tinction rests upon equally slight color characters supple- 

 mented, it is true in some cases, by present geographical 

 separation of habitats. 



* ' It is an interesting fact that the embryonic young of the 

 brown-tipped form differ more from the young of the others 

 than do the parent shells. The adult of the brown-tipped var- 

 iety is to be distinguished only by the light-brown color of 

 the apical whorls. The embryo shells have the lower half of 

 the basal whorl of a deep reddish-brown. Evidently the brown 

 tends to fade out as the shell matures. It is probable that in 

 time the deep brown apex will become a permanent character 

 of the adult of this form as it now is an invariable character 

 of the young. 



