258 PNEUMONOBRANCHIATA. 



4. Consequently, he adds, the animal of Planorbis 

 is essentially dextral, like the shell. 



5. The sinistral position of the orifices of the 

 Planorbis is the same exception in the Pneumobran- 

 chous Mollusca, as the sinistral position of the gills 

 of certain Ptenobranchous Mollusca is amongst the 

 other genera of the order. 



Mr. Benson observes, — " The animal is considered 

 as sinistral ; but if the shell be viewed as such prac- 

 tically, and placed with the side which would in a 

 sinistral shell be accounted the apex, it will be 

 found that the animal is on its back, and that it will 

 have to twist its body half round, in order to gain 

 the ground with its foot ; and that, in order to creep 

 with any ease, it must reverse the position of the 

 shell. This is more especially observable in the 

 flatter and more oblique mouthed species." Mr. 

 Benson considers that face as containing the apex, 

 which is contiguous to the back of the animal. This 

 side may invariably be known in Planorbis by the 

 greater projection of the lip in that part, by the 

 deeper depression of the central umbilicus, and by 

 the more considerable involution of the whorls, occa- 

 sioning the greater depth of the suture. (Jonm. 

 Asiat. Soc. Beng. 1836, p. 744.) 



* Jflwrls not keeled, rounded above and below ; spire 

 Jlattish or slightly concave. 



100, 1. Planorbis corneus. Horny Coil Shell, (t. 8. 



f. 95.) Shell nearly flat above, deeply umbilicate 



beneath, showing the convex whorls ; whorls six, 



rounded, striated ; aperture semicircular. 



Planorbis corneus. Drap. p. 43. t. 1. f. 42 — 44.; 



