DEAN : PERIODIC VARIATION IN I.IMN.?!A PEKEGKR. 49 



I. — The Annual Forms : Unrestricted Localities. 



These forms may be considered to be, to a certain extent, fixed, 

 although individual variation is present. 



A number of localities in the south of England as well as several 

 of the North Lancashire localities have been placed under observa- 

 tion. The results all tend to show that the average life of all the 

 smaller forms is a matter of from 13 — 16 months. The breeding 

 season seems to be annually fixed, but may be early or late according 

 to the nature of the locality ; it may be short or prolonged, but is 

 more often a short period of a few weeks' duration. More frequently 

 the breeding season is in the early summer, and the young pereger 

 grow during the summer and autumn a shell varying from 3I — 3!^ 

 whorls. Then, as in all the groups, a winter check-period of about 

 three months intervenes, lasting till the end of February, when no 

 growth of the shell goes on. The next season, the reproductive 

 period again comes on, and the final quarter or half whorl is added 

 to the shell. The mature pereger then rapidly die off, giving place 

 to the young of the new generation. But, probably owing to the 

 extreme cold of some of the more northern waters, it is not uncom- 

 mon to find instances where the breeding season is deferred to a 

 short period in August, and it is only an exceptionally hot summer 

 that has any effect on the regularity of that period. 



Most of these forms are very small, with perhaps an average size 

 of II — 12 mm. length. The spire varies only slightly, the shells 

 ranging through the smaller lacustrine forms to the more acuminate 

 iruncatula-Mko. shells of the limestone wells. It will be remembered 

 that at these latter localities the conditions are often as constant for 

 a considerable period as those obtaining in the larger water-areas. In 

 all these shells the one definite winter growth-check is a marked 

 feature. Shells from normal localities show this growth-mark at the 

 end of the third or between the third and fourth whorl. Shells from 

 a retarded locality show it generally at the completion of the second 

 whorl. It is extremely rare for individuals, even at a late locality, 

 to survive a second winter, provided the reproduction period has been 

 given normal conditions. 



Before going on to consider in detail the phases in life-hiStory of 

 the Biennial forms, it may be of interest to notice the effect of ex- 

 ceptional circumstances in the case of a habitat which, under normal 

 conditions, possesses all the essential features of an unrestricted 

 locality. This habitat is a small artificial trough placed at the base 

 of a limestone cliff and receiving water from fissures in the limestone. 

 The form of shell is similar to the shells of the limestone wells 

 described above. The animals breed in August, and attain maturity 



O 



