2 XilK CHICAGO ACADEMY OF SCIENCES. 



margins, as in haldetnani, or wide, rounded and even patulous, as in 

 columella and auricularia. It varies from one-fourth to four-fifths the 

 length of the entire shell. The aperture is usually angulate above and 

 may be rounded, ovate or effuse below. 



The Outer lip (?) is always thin at the margin, but in many 

 species develops a heavy varix just within the aperture, a trifling dis- 

 tance from the edge. This is (apparently) notably developed only in 

 those species which bury themselves in the mud during times of 

 drought. These varices show on the outside of the shell as white or 

 black longitudinal bands, and they vary in number from two to six or 

 more. They may be observed in such species as capcrata, palustris, 

 clodes, rcflexa, parva, etc. 



The Inner lip (6) is reflected over the columellar region, a por- 

 tion being closely appressed to the parietal wall (5) while the lower 

 part may be tightly appressed to the columella, thus forming a dis- 

 tinct ascending columellar plait (stagnalis, auricularia, palustris), 

 tightly appressed but not forming a plait (megasoma, haldemani, colu- 

 mella) or reflected and turned back, forming a broad expansion which 

 overhangs the umbilicus (bulimoides, techella, capcrata, cmarginata). 



The Axis or pillar (in the upper whorls) may be gyrate or 

 twisted in the form of a spiral (stagnalis) , or it may form a straight 

 smooth column, as in bulimoides and obrussa. All gradations occur 

 between these two extremes. 



The Umbilicus (8) may be a mere chink or narrow slit behind 

 the reflected inner lip, or it may be a wide and deep hole, overhung or 

 emargined by the expanded inner lip. In no case is it a hollow tube, as 

 in many of the groups of fresh-water and land shells. In one species 

 (Galba randolphi) a bristle may be passed to the penultimate whorl, 

 but not beyond, except in abnormal specimens. In almost all individ- 

 uals the umbilical opening is closed by the columellar axis of the pre- 

 ceding whorl. A difference should be made between the umbilical 

 chink, which includes those shells in which the hole is but one whorl 

 deep, and the true umbilicus in which the whorls revolve around a 

 hollow tube, open to the apex. The Lymnseas belong to the former 

 type. 



The Nucleus or Protoconch is a small rounded knob of from 

 1%. to V/z whorls, and is without distinct sculpturing. Under a glass 

 the surface appears to have a satin-finish appearance. The nucleus 

 varies slightly in form in many of the species (see plate XLIX). In 

 a few species (umbilicata, for example) the nuclear whorls appear to 

 be very faintly substriate. 



