158 MOLLUSCA. 



this naturalist had been deceived by some of the parisitical 

 leeches which infest the aquatic pulmonifera, and that, instead 

 of breathing by means of tubular gills, the animal of the L. 

 auricularia possesses, like those which it resembles in other 

 characters, a pulmonary cavity. 



GENUS PHYSA. Pillar-lip destitute of a fold, and the 

 whorls are sinistral. (Zool.Journ. vii. 363.) The external ap- 

 pearance of the animal is similar to the Limnea; but the mar- 

 gin of the cloak is loose, divided into lobes, and capable of 

 being reflected over the surface of the shell near the mouth. 

 This genus was instituted by Draparnaud. The Bullafonti- 

 nalis of British authors is regarded as the type of the genus. 



GENUS APLEXA. Pillar-lip, with a fold. This genus 

 was instituted by us for the reception of the Butta hypnorum 

 and rivalis of British writers. * The shell is more produced 

 than in the Physa. The cloak of the animal is incapable of 

 being reflected on the shell, and its margin is destitute of lobes. 



GENUS PLANORBIS. Cavity of the shell entire. This is 

 another sinistral genus ; the vent, pulmonary cavity, and 

 sexual organs, being on the left, and the heart on the right 

 side. The P. corneus, the type of the genus, pours forth, 

 when irritated, a purple fluid from the sides, between the foot 

 and the margin of the cloak. 



GENUS SEGMENTINA Cavity of the shell divided. Ex- 

 ternally, the shell appears similar to Planorbis ; but inter- 

 nally, it is divided by testaceous, transverse partitions, into 

 several chambers, which communicate with each other by 

 triradiated apertures. It is uncertain whether the animal 

 is to be considered as dextral or sinistral. This genus was 

 instituted by us several years ago, for the reception of the 

 Nautilus lacustrisof Lightfoot, first described and figured ia 

 Phil. Trans, vol. Ixxvi. p. 160. tab. 1. f. 1, 8. 



