470 



LIMN^IDiE. 



They are strictly aquatic in their liabits, abounding in the small 

 quiet streams and stagnant ponds, feeding exclusively on vegetable 

 substances. They usually come to the surface to breathe the free 

 air, but their organs of respiration must be adapted, in some species 

 at least, to breathing through the medium of water, as they are oc- 

 casionally found in circumstances precluding any possibility of an 

 approach to the surface. 



Their eggs are laid in clusters, surrounded by a gelatinous matter. 



Many of the species possess the power of gliding along the sur- 

 face of the water, shell downwards, and letting themselves down by 

 means of a gelatinous thread. 



From the fact of my finding young individuals only in the spring, 

 and numerous dead full-grown shells during the late autunm and 

 winter, I presume they arrive at maturity in one season. They are 

 active during the spring, summer, and autumn, but bury themselves 

 in the mud during winter, at least in the Northern States. 



The LimncBidcc have been grouped by some authors according to 

 the number of their horny jaws, but in the present stage of knowl- 

 edge of them it seems to me preferable to adopt that division into 

 sul)-families based upon the form of the shell, which is found to be 

 spiral and elongate, spiral and flattened, or non-spiral and simply 

 patclliform. 



The shells of some of the various genera present consideral)lc dif- 

 ference in form, but their characters are not as well marked or reli- 

 able as in the Helicidce. 



FiK 722. 



Auitnal of L. me^usoma 



Oeiiiis L,IItIN^A, Lamarck. 1799. 



Tentacles flattened and triangular. Man- 

 tle with the front edge thickened. Foot short, 

 rounded. Shell dextral, spiral, oblong, trans- 

 lucent, horn coloi-ed ; spire acute, more or less 

 {produced, last whorl ventricose ; aperture large, 

 Avide, rounded in front ; inner lip with an ob- 

 li(pie fold ; outer lip simple. 



Jaws three, smooth ; one upper, large, trans- 

 versely oblong or ovate ; two lateral, rudimen- 

 tary, narrow, convex. 



Lingual membrane (of L. columella) broad, 

 teeth crowded, numerous ; central narrow, 



