No. 473] MUTATION IX MOfJA'SCA 



331 



from no teeth through one, two, to three teeth. Some of tin- forms 

 of Polyoryra may be perforate or imperforate i I'nh/,/i/r(i mnitodon, 

 for example). In Valvata (Fig. 3) tlie variation to Ix' still 



carinae; for example, I'alrafa fricariiiata ha> three cai'ina' I ; var. 



So also with Valvata bicarinata, which has two carina' (o) while 

 the variety normalis has three carina (4). \' i ri para contrdoides 

 is another example in point, the typical form being l)aii(le(l, while 

 a variety is without bands. These variations would seem to con- 

 form to de Vries's retrograde varieties, differing from the parent 

 species in the absence of one or two characters. The majority of 

 the latter examples are true varieties, lacking some characteristics 

 of the type form, while the variations of Lymnaea are mutations 

 combining most of the characteristics of the parent form. It is 

 to be noted, however, that these two conditions overlap each other, 

 so that no sharp and fast line can be drawn between them. 



Some of the paleontologists have hailed de \'ries's theory with 

 delight, for they say that it is only in the liuht of such a theory 

 that the sudden appearance of marked tyi)es in certain ancient 

 faunas becomes intelligible. May it not also account for the 

 finding of certain new species in regions supposed to have been 

 thoroughly explored? :\Iay it not also account for the sudden 

 disappearance of certain species, the mutations dying out but 

 the parent form still continuing? A case in point is Ijjmmra 

 shurileffi described from an artificial pond at Weatogne. Hartford 

 Co., Connecticut. This species was found in large numbers, 

 together with a new variety of Planorbis [P. circum.striatus Tryon). 

 The finding of this Lymnjea is thus described by Mr. Tryon: ' — 



"The circumstances under which this and the following species 

 were found are so peculiar that it is with great hesitation that I 

 have ventured on a description of eitiier of theni. 'i'hat new 

 species of these shells should exist nndeteete*! in sections of the 

 United States which have been so well explored by assiduous 

 naturalists would be surprising; but in the present instance the 

 almost irresistible supposition is, that these species are of very 



'Amer. Joum. Conch., vol. 2, p. 112. 1S66. 



