50 THE NAUTILUS. 



Two adductor muscles, kidney-shaped in section, one on each side, 

 replace the usual columellar muscle. 



The radula has 18, 1, 18 teeth, arranged about as in Lymnaa. 

 Centrals with a single cusp, the laterals bicuspid, the outer cusp 

 smaller. The transition teeth have four or five cusps. The mar- 

 ginal teeth are low, wide and separated, with four or five cusps. 

 PI. Ill, fig. 6, teeth of A. alabamensis. 



There is a short false gill hanging in the pallia! cavity. 



The shell has some resemblance to Crepidula and Latia. Lepy- 

 rium and Pompholyx are also slightly similar; but the resemblance 

 is no doubt superficial. Pompholyx, like Amphigyra, is sinistral 

 with an ultrasinistral shell. It has no plate or lamina across the 

 visceral cavity. The soft anatomy of Amphigyra, so far as worked 

 out, seems to show most affinity with the Ancylidse. 



The deck of Amphigyra is present at all stages of growth ob- 

 served, in young as well as mature shells. In Gundlachia no septum 

 is developed until a period of hybernation or aestivation is reached. 

 The shell is more solid than that of Ancylus or Gundlachia. 

 Amphigyra alabamensis n. sp. PI. Ill, figs. 1, 2. 



The shell is shaped like a convex Crepidula, closely, finely and 

 sharply striate spirally, and of a pale yellowish-corneous tint. The 

 last whorl flares in a raised ledge at the baso-columellar region, the 

 back being very convex. The spire is slightly sunken, depressed. 

 The raised parietal margin of the lip is abruptly kinked where it 

 passes across the preceding whorl. The columellar plate or deck 

 extends over nearly one-third the total transverse length of the 

 aperture. Alt. 1.1, diam. 2 mm. 



Wetumpka, Alabama, on the under surfaces of rocks in swift 

 water. 



Genus Neoplanorbis n. gen. 



The shell is minute, subdiscoidal, nearly flat above, convex below, 

 perforate, carinate at the periphery, composed of about two rapidly 

 enlarging whorls, the apex impressed and turned in. The aperture 

 is very oblique, wider than high, a little dilated at the base. Peris- 

 tome thin, not continuous, the columellar margin straight and 

 broadly dilated, somewhat thickened within. 



The dentition and so far as known, the soft anatomy, is similar to 

 Amphigyra. 



Type N. tantillus. 



