742 Description of Land and Fresh-water Shells [Nov. 



tegente ; supra convexa, versus apicem planata, apice concavo, omni- 

 bus anfractibus satis apparentibus : periphseria obtuse angulata. 

 Diam. 0.3 poll. 



This shell, belonging to the same division of Planorbis as the Bri- 

 tish species PI. fontanus, (PL nitidus, Lamarck,) in which the whorls 

 on the inferior side are nearly covered by the succeeding ones, may 

 easily be distinguished from that species, which it resembles also in 

 colour, by the greater convexity of the last whorl towards the peri- 

 phery, and by the slope towards the penultimate whorl, on the under 

 side ; while in PL fontanus the convexity is next to the penultimate 

 whorl, and the slope tends towards the circumference. The disposi- 

 tion observed in PL umbilicalis occasions a great concavity on the 

 under side of the shell. The superior margin of the aperture, as in 

 most of the depressed Planorbes, projects much behind the inferior 

 margin, occasioning a great obliquity from the plane of the axis. The 

 North American species PL deflexus and PL exacuens of Say, more 

 especially the latter, have an affinity to this shell, which is probably 

 an inhabitant of the streams of Silhet. 



20. Planorbis trochoideus. Testa, quasi dextra diaphana, subtrochi- 

 formi, supra glabra, rotundato-convexa, apice concavo-depresso quasi 

 umbilicali ; anfractibus omnibus pariim apparentibus, satura excavata 

 divisis ; infra radiatim striata, truncata, planata, umbilico contracto. 

 Anfractu ultimo majori reliquos amplectente, supra pene, infra omnino 

 obtegente ; periphseria acuta. 



The last whorl altogether conceals the other whorls on the lower 

 face of the shell, leaving merely a contracted umbilicus in the centre, 

 towards which the flattened surface declines. On the upper side only 

 a small portion of the depressed spire is shewn, the whorls being rather 

 compactly wound. The species differs from all others hitherto de- 

 scribed in the singular truncated form of the inferior surface, and in 

 the circumstance of the last whorl winding closely round the axis, as 

 in many Helices and Trochi, without the intervention of the preceding 

 whorls, none of which are visible in the very contracted umbilicus. 

 The only shell of the genus which bears any very near resemblance 

 to it in the formation of the under side, is PL fontanus of British 

 authors, (PL nitidus of Lamarck,) the umbilicus of which is unusually 

 contracted when compared with other species of the genus, but still 

 exhibits within its vortex the whole of the preceding whorls, and so 

 does not deviate from the recorded generic character ; while the spe- 

 cies now brought to notice renders nugatory, as far as it is concerned, 

 two important characters of Pianorbis, viz. " anfractibus omnibus 

 utrinque conspicuis," and " apertura ab axe remotissima." For the 



