he 
LYMN=IDZ 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 gomg 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 calcified shell. As the 
margin expands or contacts over the still viscous secretion, the orna- 
mentation of the mantle edge, cilia, papille, 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 striz, the re- 
volving threads and similar ornamentatioa developed on the suriace of 
a fresh water gastropod. The transverse sculpture, usually known as 
incremental lines, arises from the periodicity of secretion, while nb- 
bing or spinose ornamentation originates in a periodic turgidity of the 
mantle (how induced normally is not known) which rhythmically ai- 
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? 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 Lymnea emarginaia irom Eagle Lake pomts to 

considerable fluctuation in the volume oi 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 
“2Proc. U. S Nat. Mus. XXII, p. 135. 
