OF NORTH AMERICA. 31 



position of the shell substance and epidermis takes place from the sur- 

 face and the edge of the mantle. The process is not absolutely con- 

 tinuous, but is carried on at more or less frequent intervals when the 

 animal is in a state of rest. At times when deposition is going on, the 

 margin of the mantle is in a more extended state than usual, reaching 

 to a point where the extremely thin and delicate membrane is- in con- 

 tact with the extremest margin of the already formed shell. The gland- 

 ular epithelium of the edge of the mantle secretes less lime than that 

 of the surface behind it, and is chiefly responsible for the periostracum 

 of the shell, while the rest of the mantle has the task of secreting the 

 more limy matter which makes up the bulk of the calcined shell. As the 

 margin expands or contacts over the still viscous secretion, the orna- 

 mentation of the mantle edge, cilia, papillae, fringes, etc., everything 

 which by its form or bulk varies the flatness of the filmy membrane 

 itself, mechanically influences the form of the surface over which it 

 passes, as the teeth of a rake leave shallow furrows over the gravel 

 of a garden walk. Essentially in this way are the spiral stria?, the re- 

 volving threads and similar ornamentation developed on the surface of 

 a fresh water gastropod. The transverse sculpture, usually known as 

 incremental lines, arises from the periodicity of secretion, while rib- 

 bing or spinose ornamentation originates in a periodic turgidity of the 

 mantle (how induced normally is not known) which rhythmically af- 

 fects that organ, and by its tidal rise and subsidence causes the shell 

 secreted during such epochs to be more inflated or capacious than 

 at the corresponding intervals. These features and modes of growth 

 can be observed in an aquarium with the more common fresh water 

 gastropods." 



Dr. R. E. C. Stearns 2 has recorded the case of a locality in Min- 

 nesota known as Eagle Lake, of which he says, "The testimony of the 

 numerous examples of Lymncea emarginata from Eagle Lake points to 

 considerable fluctuation in the volume of the water one season com- 

 pared with another, and the more northerly Eagle Lake to the influence 

 of extreme cold, or alterations of temperature conditions as related to 

 volume of water in some seasons or years, as well as possibly alka- 

 linity or some chemical fluctuation due to diminished volume of water 

 at times, or briefly, to fluctuations in the quantity and temperature of 

 the water and the chemical quantity, or proportions of the same." 



The Galba emarginata from Eagle Lake are very variable, the 

 spire being long or short, the body whorl rounded or shouldered, aper- 

 ture rounded or patulous, surface malleated or simply lirate, and sutures 



2 Proc. U. S. Nat. Mus. XXII, p. 135. 



