Manchester Meuioirs, Vol. Ivii. (191 3), No. 10. 9 



Now it is evident from a careful examination of a large 

 number of specimens that shells with a conspicuously flat 

 apex are rare, as are also those with a very pointed apex, 

 while intermediate types are common, whence we are 

 justified in the conclusion that the variation in the rate of 

 assumption of the turreted character is again a continuous 

 variation. 



It is, of course, obvious that this variation must be 

 evident from an examination of the young shells them- 

 selves, but it will be convenient to defer this enquiry 



Fig. 6. Section of apex of shell of mean height, 

 showing early development of very 

 strong carination. 



until some of the other variable characters of the shells 

 have been considered. 



Variable Characters other than Height. 



(i) Carination. The shells may possess two carinae 

 {i.e., ridges running along the coils of the shell), a dorsal 

 or median one, and a ventral one ; either or both may be 

 absent. These carinae are usually moderately developed, 

 occasionally being very strong or rarely quite absent. 

 Between these limits every variety may be found, as shown 

 on PL /., Fig. 2 A-D, E-R, l-L. They are, therefore, contin- 



