SKENEA. 159 



This and the succeeding species are two of the most mi- 

 nute of our British shells, and bear a considerable resem- 

 blance to the genus Planorhis. For there is no projecting 

 spire, but the shell is discoid, involute, and sunken a little 

 in the middle, both above and below. It is very thin, 

 highly polished, and of a clear semitransparent wax or ful- 

 vous horn-colour. Many individuals exhibit, likewise, some 

 radiating wavy streaks of white, and a few white lines 

 (like those of Sefjmentina) but these are possibly produced 

 by the unequal drying of the shelly matter. To the un- 

 aided eye, or even under a lens of low power, the surface 

 appears smooth, but under the microscope, numerous and 

 densely disposed longitudinal wrinkles, that seem to become 

 obsolete around the middle of the body Avhorl, and are more 

 or less indistinct in the middle of the smaller turns, are 

 clearly perceptible. There are only two and a half volu- 

 tions, that are separated by a simple but strongly im- 

 pressed suture, and rather gradually enlarge from a 

 tolerably large apex. The body is rounded at the cir- 

 cumference, but shelves rather more above than below ; its 

 height manifestly increases as it recedes from the lip. 

 The aperture, which is of about equal length and breadth, 

 and occupies more than one-third of the basal diameter, 

 resembles in shape the moon towards its full, the pro- 

 jection of the penult whorl, preventing its being quite 

 circular. The outer lip is simple, but a little thickened 

 below, and generally appears of a rather darker tint than 

 the rest of the shell ; it is not expanded, projects above 

 the whorls posteriorly, and advances a little in front. 

 A sixteenth of an inch is the usual size. 



If this be the Truncatella atonius of Philippi, the 

 animal is white, has lanceolate tentacula, with eyes on 

 (not beside) their bases, and an oblong short foot, rounded 



