T48 Quarterly Journal of Concliology. 



of the various species and the effects of any peculiar conditions 

 carefully noted. 



In studying the variation of the fluviatile mollusca, attention 

 should be given to the fact of their growth being effected in a chang- 

 ing, moving medium, and under different conditions to land shells. 



The Strepomatida inhabit by preference rapidly flowing 

 streams in mountainous districts, adhering to rocks, exposed to the 

 full action of the current, with its varying impetus, and in time of 

 floods carrying with it particles of sand, gravel, &c, the effect of 

 which is often to loosen the shells from their moorings and also to 

 remove by attrition the confervas growing on the rocks, which con- 

 stitutes their food, subjecting them to a greater or lesser degree of 

 privation. The periodical growth of the shell renders them more 

 liable to the modifying effect of these causes, as the lips are tender 

 and very fragile during the larger part of the summer months. 

 The relation of sex and form and the effect of chemical influences 

 are questions as yet imperfectly understood. 



Dr. James Lewis, of Mohawk, whose name is so honorably 

 associated with the Strepomatidae, ascribes their advent to the 

 Carboniferous epoch, and Mr. Wetherby by a study of the veget- 

 able and other organisms of that remote period infers the nature of 

 the habits of their ancestral forms and conjectures them to have 

 their analogues in the subtropical families of Vivipara, Neriti/ia, 

 Mela riia, Ceri Hi idea, &*c, and that the Strepomatida may have been 

 evolved from these species by the operation of natural causes. 



The author suggests the modern alliances of the tropical 

 Ampullaridce and Melantho, Neritina and Anei/losa, Cerithidea 

 and Tiypanostoma. 



If the suggestion of Dr. Lewis be a just one, that shells are 

 propagated down stream, then the origin of these shells or their 

 ancestral types, if still existing, will be found at the upper part of 

 the drainage and that least affected by change of level. 



Mr. Wetherby concludes this highly interesting paper by des- 

 criptions of the following five new species: — 



Lithasia plicata, Wetherby. Green River, Tenn. (PI. II, fig. 1.) 



