180 



mellar division having more digitations, usually five, than that which is 

 projected over the spire. The foot is obtusely angled in front and pointed 

 behind. It does not appear that there are any essential differences dis- 

 tinguishing the animals of the different varieties. They all advance with a 

 jerking motion." 



1. acuta, Draparnaud. 



2. alba, Turton. 



3. aurea, Lea. 



4. castanea, Lamarck. 



5. coutorta, Midland. 



6. elliptica, Lea. 



7. elongata, Say. 



Species. 



8. fontinalis, Drapnd. 



9. Georgiana, Quoy. 



10. Guerinii, Mittre. 



11. gyrina, Say. 



12. beterostropha, id. 



13. hvpnorum, Linneevs. 



14. Ludoviciana, Mittre. 



15. Mangerise, Gray. 



16. Novse-Hollandise, Blvl. 



17. Peruviana, Gray. 



18. rivalis, Sowerhy. 



19. subopaca, Lamarck. 



20. Tongana, Quoy. 



21. truncata, Fe'russac. 



Figure. 



Physa Mangeri,e. PL 17. Fig. 91. Shell, showing its triangularly 

 ovate form, and dark porcellanous shining surface. 



Genus 4. PLANORBIS, Guettard. 



Animal ; having a stout broad proboscis-like head, with two slender 

 tentacles, with eyes at their inner bases ; foot short and obtuse. 



Shell ; discoid, with a depressed spire, with the apex more or 

 less sunk in a concavity ; whorls rounded, sometimes keeled, 

 with the aperture mostly semilunar ; lip simple. 



We have seen how puzzled conchologists were as to the affinities of 

 Ancylus, owing to the Limpet-like form of its shell. Similar difficulty 

 occurred originally with Planorlns ; the shell being distinguished by the 

 same peculiarity of the whorls coiling upon each other in a discoidal plane 

 as in the fossil Ammonites. Cuvier was the first to determine anatomically 

 the affinity of Planorbis with Lymncea and Physa, as suggested by their 

 similarity of habit ; and so uniform in character are its numerous species, 

 that no subgenus has been proposed worth recording. 



The animal of Planorbis is rather variable in colour, black or slate, or 

 chestnut or yellowish, or yellow dotted on a dark-fuscous ground. The 

 shell is mostly of a livid-slate or greenish-horny colour, inclining to yellow. 



