PLANORBIS. 



the umbilic to the upper side. If a dextral, dis- 

 coid Hehx (as CarocoUa albella) be placed be- 

 fore a mirror,* the reversed image will not cor- 

 respond with a Planorbis; as, in the former 

 genus, dextral or sinistral, the spire is invari- 

 ably upon the right, or upper side; so that the 

 shell of the typical section of Planorbis is 

 anomalous, whether it be viewed as dextral or 

 sinistral, the apex being upon the side towards 

 which the plane of the peritreme is directed. 

 This circumstance appears to have been hereto- 

 fore overlooked, to which may be attributed the 

 difference of opinion upon this question. When 

 Sowerby asserts (from examination of the ani- 

 mal) that Planorbis carries its shell differently 

 from dextral genera, he must have looked at the 

 apex alone; for in other respects, Planorbis 

 corneus and Helix albella correspond exactly. 

 If the apex is to determine the question; sup- 

 posing the animal to be deprived of its shell; 

 how easily it might be made dextral or sinistral, 

 by pressing the part corresponding to the apex, 

 from side to side. Planorbis parvus, deflectus, 

 contortus, vortex, and complanatus, have flat 

 shells, which show the apex on both sides, 



* The comparison may be more easily made, by using 

 a reversed Helix. 



