Es-GEKsoLL.] ZOOLOGY PLANORBIN^. 403 



course of some paloontological investigations. Concerning this matter 

 Professor Hyatt writes as follows, in a letter dated Febrnary 10, 1876: 



"These variations have been studied only with reference to the shell, 

 but the changes of form are so great in this external organ, that one 

 naKirally infers corresponding differences in the animals themselves. 

 The principal papers heretofore published upon these interesting shells 

 are but two in number: one by Hilgendorf, in the Monatsber. d. kongl. 

 Preuss. Akad. d. Wissen., 1866, uj^on the fossil forms of. FlanorMs mul- 

 tiformis; and one by M. Pire, upon Planorhis complanaius^ in the Annales 

 de la Soc. Malacol. de Belgique for 1871. The recorded information is 

 therefore scanty, and it would be a very important service to conchology 

 and paleontology if every one who has met with abnormal or distorted 

 forms, in the course of his collecting of land or fresh -water shells, would 

 make i^ublic all the information he has in connection with those discov- 

 eries. Eecords of such experiences are extremely desirable. 



" Both of the papers alluded to above are accompanied by figures, and 

 show a very remarkable series of forms, which vary from the flat spire 

 with equal umbilici, to those which are completely trochiforra, and from 

 these to specimens entirely unwound, like a wire corkscrew. I have 

 myself studied attentively the Stsinheim beds described by Hilgendorf, 

 and can confirm his results so far tis the extreme variations of form are 

 concerned, though in other respects his paper is full of erroneous state- 

 ments, especially with regard to the genetic connections and stratigraph- 

 ical distribution of the varieties. 



''I have also a very remarkable series of shells, probably belonging to 

 Valvata, which I owe to the kindness of Prof. Edward S. Morse. They 

 were collected by Prof. O. F. Hartt in marl laid dry by the drainage of 

 Lawlor's Lake in Nova Scotia. These are equal to any described species 

 in variation, some of them being actually unwound, with a perfectly 

 cylindrical outline, to the mouth of the shell. What the governing 

 peculiarities of the locality last named may have been at the time the 

 marl was deposited, I cannot say, but the condition of the Steiuheim 

 Lake during the Tertiary period, and of the small ponds, described by 

 M. Pire, resembles closely that of the localities described in your 

 paper. 



" The Steinheim Lake was evidently, as shown by Quenstedt and Fraas, 

 an isolated sheet of water about a mile in diameter. The ponds of 

 Magnee, according to M. Pire, are fed only by rain-water, but are never 

 frozen and never dry. 



''isTot only, therefore, is the occurrence of these extreme variations 

 exceptional, but they appear in localities presenting certain excep- 

 tional characteristics. These characteristics are well worth investigating, 

 since it seems as if a direct correlation existed between the extreme 

 variations of the shells, and some physical cause common to all the 

 localities in which the distorted specimens have been found. That the 

 variations are not distortions in the ordinary meaning of that word, can 

 be readily understood by any one who has studied an extended series of 

 them. The most aberrant of these varieties in Steinheim has descendants, 

 which perpetuate its peculiarities for what must have been a consider- 

 able lapse of time, forming races of greater or less importance; and M. 

 Pire inferred the sanSe fact at Magn6e, from dead shells found buried in 

 the mud at the bottom of the cisterns. I have no doubt that the reniust 

 be a vast array of similar experiences awaiting any explorer of the iso- 

 lated lakes and ponds of this country, and I hope your publication will 

 open the way for many similar observations. In no other direction can 

 we look. for more light upon the mode of origin of new races and forms, 



