PULMONATA. — oe 
x 
axis. A series of such distorted specimens of P. complanatus, taken from a pond near 
Swansea, formed, I believe, by the waste water from a steam-engine, and of a high 
temperature, is in the British Museum. These specimens have all assumed an elevated 
spiral form; and the aperture is in every case dextral. Several specimens of P. vortea, 
in Mr. Sowerby’s Museum, are similarly distorted ; and in them also the apertures are 
dextral. On the whole, the better opinion appears to be that the shell, as well as the 
animal, is dextral, notwithstanding the abnormal position of the heart and the orifices; 
and in the following descriptions, therefore, I have considered the shell as dextral, and 
I have applied the term wpper to that disc which is uppermost when the shell is placed 
with the mouth on the right side of the spectator, and the term wnder to the opposite 
disc. 
The Planorées live in fresh water; more frequently in stagnant water or standing 
pools, although, occasionally, they are found in gentle streams. They are widely 
diffused, but abound principally in temperate climates. I believe that at present there 
is not any species known as living in salt or brackish waters; and the specimens found 
in the crag formation, and described by Mr. Wood, are referred to recent species 
which are known to be pure fresh-water animals; and these shells are therefore consi- 
dered to have been accidentally introduced. 
Four species also occur in the estuarine or fluvio-marine deposits of the Eocene 
epoch: viz., P. hemistoma (Sow.); P. obtusus (Sow.); P. biangulatus (nov. spec.); and 
P. elegans (nov. spec.); but, like the crag specimens, they have, probably, been depo- 
sited there by the agency of some river: they all occur in the pure fresh-water or 
the transition formations. 
Fossil species are numerous, but they abound principally in the formations of the 
tertiary epoch; Prof. E. Forbes, however, states, (Brit. Mol., vol. iii, p. 146,) that 
representatives of the genus, differing but slightly from species still living, are found 
in fresh-water strata of even the oolitic epoch. 
No. 51. PLANORBIS EUOMPHALUS. Sowerby. Tab. XV, fig. 6 a—c. 
PLANORBIS EUOMPHALUS. J. Sowerby. Min. Con., vol. ii, p. 92, t. 140, figs. 7—9. 
= = G. Sowerby. Genera of Shells, fig. 5. 
— — Deshayes. Lam. Hist. Nat., 2d edit., vol. viii, p. 397, No. 9. 
P. testé supra sub-pland, ad peripheriam angulatd, subtus late et profundé cavata ; 
anfractibus sex, sub-trigonis, vie involventibus, transversim lineis incrementi notatis, 
aliquandoque concentricé striatis ; subtis ad marginem umbilicalem obtuse angulatis; striis 
concentricis numerosis, irregularibus : apertura per-obliqud. 
This well-known species, which at present appears to be confined to the fresh- 
water formations of England, is easily distinguished from the cther fossil Plaxordes. 
It is a large discoidal shell, nearly flat on the upper face, and presenting a wide and 
