114 On the Land and Fresh Water Mollusca 



me in the opinion that it is a variety of this species. In some spe- 

 cimens of PI. albus, the spiral striae are scarcely discernible, even 

 in a living state, and become quite obliterated in dead shells. 



10. Contortus, Linn. Mull. 



Jl. Lineatus, Walker, Jeff. 

 P. clausulatus, Fer. 

 Segmentina lineata, Flem. 



The tripartite partitions in this shell, though remarkable, are 

 scarcely sufficient to raise it to the rank of a genus. The animal is 

 exactly that of a Planorbis. 



12. Nitidus, Mull. 



PI. complanatus, Drap. 

 As the PI. nitidus of Muller evidently includes both this and the 

 last species, authors differ much in opinion as to which of them 

 should bear the name. To avoid the confusion that has hence arisen, 

 it would be better to drop it entirely, and take for this species the 

 name of lenticularis, which is given to it by some continental au- 

 thors, and is very characteristic. 



13. lmbricatus, Mull. 



Var. PI. cristatus. 



Gen. 19. Physa, Draparnaud. 



1. Fontinalis, Linn. Drap. 



2. Hypnomm, Linn. Drap. 



The difference between the animals of this and the former species 

 perhaps justifies Dr Fleming in establishing the genus Aplexa for 

 the present. A better knowledge of the other animals of this genus 

 would enable us to separate them with more confidence. 



Three other species of Physa have been described as British. 

 Two of these, Bulla (Physa) rivalis, Maton, and B. fluviatilis, Turt. 

 Mr Jeffreys supposes, with some probability, to be varieties of P. 

 fontinalis. There is every reason to believe that Physa alba, Turt. 

 is a foreign species. 



Gen. 20. Limneus, Draparnaud. 



1. St agnails, Linn. Drap. 



Var. H. fragilis, Linn. ? Mont. 

 Young, L. Scaturiginum, Turt. 



2. Palustris, Linn. Drap. 



This is a very variable species, and has been divided into three 

 or four by continental authors, upon the propriety of which division 

 it is not easy to decide. There is a reputed variety inhabiting this 

 country (var. /3. Jeff.) which seems to have some claim to the rank 

 of a species. It is found on the margins of rivers in different parts 



