Shells of Common Occurrence. 155 



fourth, to one- fifth; P. nautileus, one-eighth to one- 

 tenth; P. marginaiusy five-eighths; P. carinatus, 

 P. spirorbis, one-fourth; and P. contortus, one-fifth 

 of an inch in diameter, respectively; whilst P. 

 vortex is usually only from three to four lines, and 

 P. nitidus two and a-half lines. These shells are 

 found in the slow rivers, pools, and stagnant waters 

 of England. The P. vortex does not actually reside 

 in the mud, but on its surface ; but more especially 

 occupies the stems and leaves of plants, both in 

 and out of the water, retiring into recesses and 

 cavities in the banks formed by the plants or their 

 overlying stems or leaves. In point of fact, the 

 mud shells, par excellence, are the group of Lim- 

 ncecea, of which there are given seven examples 

 (Figs. 79 — 86). The family is wide-spread, the 

 shells are fragile ; and Mr. Lowe has noticed that 

 the property of walking upside down on the ceiling, 

 appertaining to the house-fly (Musca domestica), 

 has been introduced by the Limngeus into the 

 watery regions, " for it as easily crawls upside down 

 on the surface of the water basking in the sun, as 

 it moves in the ordinary manner on the surface of 

 the mud." The lake and river limpets, Ancylus 

 oblongus and A. ftuviatilis (Figs. 87 and 88) are 

 small breathing animals; and the minute sedge 



