TAYLOR: VARIATION OF LIMN^A PEREGRA. 287 



The opposite extreme — the attenuation of the substance of 

 the shell — is one of the effects ascribed to the inhabited water 

 being either much warmer or much colder than the ordinary- 

 temperature, the variety ihermalis, which is very thin and trans- 

 parent, living in the warm springs of France, which are said 

 to attain a temperature of 95 '^ Fahr. The var. glacialis exhibits 

 as regards the tenuity of its shell substance the same peculiari- 

 ties as var. thermalis, but lives habitually in water but little re- 

 moved from the freezing point even in summer. Thus both these 

 extreme conditions seem to have equally the effect of retarding 

 the development in size of the shell. 



Deep water is said to have a similar effect judging from 

 Nilsson's description of the L. baltliica, which is said to live at a 

 depth of from 24 to 36 feet in the brackish-water of the Baltic, 

 and Mr. W. Thompson, the a])le Irish naturalist, ascribed the 

 peculiar delicacy of the specimens of the var. lacustris from 

 Lough Neagh, to the circumstance that they habitually lived in 

 the still depths of that lake, and were only to be found on or 

 near the shores owing to fortuitous circumstances. 



It is, however, conceivable that causes other than those 

 mentioned here, such as a scarcity or abundance of calcic car- 

 bonate may produce the same attenuation or thickening of the 

 shell substance ; but the more usual result of such conditions 

 are the erosion or the incrustation of the shells, according to the 

 deficiency or excess of the calcareous salts in the inhabited 

 water. 



Many of the specimens which are recorded from time to 

 time in the scientific journals as Jeffreys' var. pida^ only show 

 markings which are the effect of slight injuries to the mantle 

 margin, resulting in a defective or thinner secretion of epidermis 

 or perhaps a total failure to secrete it in the injured portions, so 

 that the calcareous portion of the shell is more or less exposed 

 in a spiral line or lines. The Rev. Dr. Norman, however, 

 assures me that the var. pida from the original locality do not 

 owe their peculiarity to this cause. 



