loS JOURNAL OF CONCHOLOGY, VOL. 9, NO. 4, OCTOBER, 1898. 



Some are stunted and turreted, others are elongated and slightly 

 scalariform, while one is beautifully banded with white. 



Valvata cristata. — This species is occasionally distorted in the 

 last whorl, which is slightly bent downwards. 



Limnaea peregra. — There are two rather small forms of this 

 species living together in the ditch. One, the larger of the two, has a 

 produced spire and is of the normal yellow colour, probably var. acu- 

 minata ; the other is of a deep red colour, occasionally blotched 

 with yellow on the body-whorl. The majority of these have in addi- 

 tion to the abnormal colouring a very pronounced growth-check, 

 about half-way round the body-whorl. The shell is rather solid till 

 it reaches this check, when a much thinner growth takes place, appa- 

 rently proceeding from the inner-edge of the old growth, as it leaves 

 a ridge on the outside, which juts out prominently at the base of the 

 shell. A possible cause for this change of growth may be the flood- 

 ing of the ditch into the pastures during the heavy rains of November 

 and December last, when building operations would in all probability 

 be stopped for a time. Then, when the water subsided and building 

 was resumed, the shell-forming ingredients in the ditch may have 

 become so diluted with the flood-water that a thinner growth resulted. 



From the patches of yellowish sediment which are to be seen in 

 some parts of the ditch, I imagine the water contains a fair percent- 

 age of iron, which might cause the peculiar red colouring of the 

 shells, though why it should not also affect the other form of L. 

 peregra is not easily accounted for. 



Apart from mollusca, the ditch is also full of newts, sticklebacks, 

 caddis-larvae, water-beetles, water-spiders, etc., and as might be 

 expected, to balance such a large amount of animal life, it is covered 

 over with various water plants, the chief of which (where the abnormal 

 forms occur) are a broad-bladed grass, Poa fluitans, also Callitriche 

 verna, and Ranunculus aquatilis. 



Most of the ditch is dried up in hot weather, and the rest has but 

 a few inches of water left in it. I should think, therefore, that one 

 chief cause of the production of the abnormal P. spirorbis is that 

 suggested by Mr. J. W. Taylor, 1 viz. : " That when the water is nearly 

 dried up, the efforts of the creatures in forcing their way through the 

 thick mud in which they are sometimes left partially embedded, to 

 again reach the water, may easily cause an alteration in the direction 

 of a new shell growth, if at the time in course of formation." 



1 Monogr. Brit. L. F. W. Moll., part 2, p. 118. 



