50 JOURNAL OF CONCHOLOGY, VOL. 13, NO. 2, APRIL, I9IO. 



the following year. During the wet summer of 1907 the tank-water 

 was in constant flow. The pereger were of a dwarf size and did not 

 deposit egg-masses till the summer of the year following, and, having 

 passed through an additional growth-period, by this time the shells 

 were quite unlike any taken previously. 



The important point here is the possible relationship of climatic 

 influence with development, and consequently with the vital question 

 of reproduction. Unfortunately for the simplification of a theory, 

 however, the clue to a fuller solution seems to begin and end here, 

 for we find in similar localities a dissimilar state of things for the 

 same period. 



II. — The Biennial Forms: Restricted Localities. 



A. — Simple alternation. 



In the first place, we may take as a generally accepted fact that the 

 larger forms are usually longer-lived. What is the life-history ? 

 Observations show that these large forms exceed the average life of 

 the smaller forms by ten to fourteen months, and, what is more 

 remarkable, that these larger forms may make their appearance one 

 year and not the next. Such periods may be regularly alternate, 

 year in and year out — in other words the variation may be biennial. 



The fuller explanation of the phase will be given by the life-history. 

 Egg-masses are deposited in May and June, and the yoMn% pereger 

 grow by the late autumn a shell of two complete whorls. There is 

 no evidence of a reproduction period the summer following, but the 

 steady addition of shell-growth is maintained until by the late autumn 

 a shell of four complete whorls has been formed. In the early 

 summer of the third year the final growth is added, and reproduction 

 takes place. By the end of July the mature individuals have died 

 off, and the new generation has taken their place. An examination 

 of the matured shells shows the two distinct growth-checks at the 

 completion of the second and fourth whorls. 



It is, perhaps, unfortunate that only one instance of this phase 

 has presented itself, but the example is valuable as showing the 

 simplest form of periodic variation. The form of shell is intermediate 

 between the annual types and the larger ovate and acuminate forms 

 of the following groups. 



B. — Reverting alternation. 



So far the matter has been fairly simple. There has been through- 

 out the constant appearance of a racially distinct type, and the first 

 stage towards development is shown in the case of simple alternation 

 just given. 



But there is another feature present in the life-history of most of 

 these \2agtx pereger, and it is from this stage that the subject becomes 



