Manchester Memoirs, Vol. Ivii. (191 3), No. 10. 13 



ing which it exhibits, and the form of the aperture varies 

 accordingly. 



In addition to such modifications dependent on other 

 variable characters of the shell, the terminal portion of 

 the last whorl of the mature shell is frequently deflected 

 in a downward direction, being at the same time slightly 

 enlarged and less extensively in contact with the preceding 

 whorl. The aperture is hereby increased in size and made 

 less angular. In extreme cases these changes may be 

 carried to the extent of a very small portion of the last 

 whorl, becoming quite free and almost completely circular 

 in section. It is perhaps justifiable to regard this tendency 

 to an ultimate decrease in involution with increased circu- 

 larity of section as a gerontic reassertion of the ancestral 

 type of the shell. The important fact, however, is that, 

 whatever significance may be attached to any of the 

 various modifications in form of aperture, all the changes 

 of this character, as of all the other characters of the shell, 

 take place by insensible gradations, and so cannot be 

 made diagnostic of any subdivisions of the species which 

 it might be desired to erect. 



Variations i?i the Form of the Yo7ing Shell. By far 

 the most surprising fact with regard to this remarkable 

 species is the circumstance that at a very early stage of 

 growth the shells assume a remarkable diversity of form, 

 while these diverse young may develop into adults, which 

 are almost indistinguishable as regards their later-formed 

 portions. Shells no more than i mm. in diameter may 

 appear so dissimilar that under ordinary circumstances 

 they would certainly be regarded as distinct species ; yef 

 they may develop into quite similar adults. Were isolated 

 specimens only available for study, this would naturally 

 be regarded as a case of heterogenetic homoeomorphy — 

 i.e., the diversity of the young stages would be taken as an 



