Shells of Common Occurrence. 155 
fourth to one-fifth; P. nautileus, one-eighth to one- 
tenth; P. marginatus, 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 Lnm- 
neecea, 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 Limnzus 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. fluviatilis (Figs. 87 and 88) are 
small breathing animals; and the minute sedge 
