PULMONATA. 99 



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. vortex, 

 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 upper to that disc which is uppermost when the shell is placed 

 with the mouth on the right side of the spectator, and the term under to the opposite 

 disc. 



The Planorbes 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. hi, 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. Sowerbg. 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. 



— — Deskayes. Lam. Hist. Nat., 2d edit., vol. viii, p. 397, No. 9. 



P. testa supra sub-pland, ad peripheriam angulatd, subtus late et pro/unde cavatd; 

 anfractibus sex, sub-trigonis, vix involventibus, transversim lineis incrementi notatis, 

 aliqiiandoque concentrice striatis ; subtus ad marginem umbiZicalem obtuse angulatis; striis 

 concentricis numerosis, irregidaribus : aperturd 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 other fossil Planorbes. 

 It is a large discoidal shell, nearly flat on the upper face, and presenting a wide and 



