Manchester Memoirs, Vol. Ivii. (191 3), iV^^. 10. 7 



regarded as an ontogenetic recapitulation of its phylogeny. 

 But it is very important to notice, first, that the change 

 occurs quite gradually ; secondly, that it is continued 

 throughout the adult stages. The first observation carries 

 with it, as a necessary corollary, that the increasing height 

 has been acquired during the past history of the species as 

 a similar gradual process. There can be no question of the 

 turreted form having arisen as a " mutation," else some 

 trace of the fact must be preserved in the ontogeny. The 

 continuation of the changes throughout life is open to a 

 variety of interpretations, but at any rate shows that there 

 is no defined form to which the species attains. 



During growth, the turreted character is thus gradu- 

 ally assumed by each individual, but to varying degrees, 

 whereby the great variety of adult forms arises. In addi- 

 tion, however, to the varying degree in which this 

 character is finally expressed, there is an independent 

 variation in the rate at which it is assumed by different 

 individuals. That is to say, the height of the shell at a 

 given stage of growth cannot be taken as a measure of 

 the height to which it will attain when fully grown. This 

 is best seen by taking a group of adult shells of about 

 mean height (form G) and examining their apices. It is 

 then found that some have a much flattened apex, due to 

 the fact that the shell maintained its discoid mode of 

 growth during the formation of several complete whorls. 

 In other cases the apex is much "sharper" (z>., more 

 conical), owing to the much earlier assumption of the 

 turreted condition. Such cases are shown in PL II., 

 Figs- 5 A-C, 6 A, P.. In Figs. 6 c, D, of the same plate 

 similar variations are shown in much flatter shells. They 

 occur, in (act, in shells of all heights, except purely 

 discoid types, which cannot, of course, express this 

 variation. 



