1. GENERAL MORPHOLOGY OF THE LYMN^AS. 



1. THE SHELL (PLATE II, FIGURE A). 



The SHELL in the Lymnaeids is typically elongated, with a pointed 

 spire and more or less turreted whorls. The shell may be as thin as 

 paper, (haldemani, columella) or, thick and solid (megasoma, cock- 

 erelli). It is usually higher than wide. The whorls are tightly coiled 

 about a central axis, forming a characteristic columella. The contour 

 of the shell varies from distinctly globose (sonomcensis, emarginata 

 wisconsinemis) to very much elongated (haldemani, reflexa). This 

 varying contour is well shown in several species, two of which are 

 mentioned below. 



Elongated. Depressed. Globose. 



bulimoides techella bulimoides cockerelli bulimoides cuckerelli (typi- 



(var.) cal). 



emarginata canadensis emarginata mighelsi emarginata wisconsinensis 



The shell is normally dextral in America, .but in the Hawaiian 

 Islands several sinistral species occur. Rarely a sinistral individual 

 will be found, one such having been seen in the collection of Mr. Henry 

 Hemphill (Galba obrussa), and one in the collection of Dr. W. A. 

 Nason (palustris). 



SIZE: The size ranges from the tiny Galba dalli, which is 4 

 millimeters in length, to the giant Lymnaa stagnalis, which attains a 

 length of 60 millimeters. 



The SPIRE (1) varies in the different species, in many being acutely 

 pointed and elongated, in some acute and depressed, in others broadly 

 dome-shaped and in a few flatly depressed. All gradations occur be- 

 tween these extremes. 



THE WHORLS (3) may be very flat sided, as in exilis and halde- 

 mani, or very convex and obese as in techella cockerelli and emarginata 

 zvisconsinensis. The SUTURES (4) vary accordingly, being just per- 

 ceptible in some species (haldemani) and deeply impressed or even 

 channeled in others (auricularia, randolphi). 



The APERTURE (2) is usually ovate, elongate-ovate or rounded, 

 but in a few species is much elongated and narrow. There are no 

 teeth or other obstructions, but a strong plait is frequently developed 

 on the columella. The aperture may be long and narrow, with parallel 



