MONSTROSITIES OP LITTORINA RUDIS. J 95 



by crowding in a later stage. .This theory is very interesting, but 

 it cannot be applied to the Fleet shells, for all their early stages are 

 proper and regular ; while the causes which make them monstrous 

 seem only to affect them later in life and after their deposition by 

 the animal. Thus the cause must, I think, be sought in external 

 influences. The shore is a well sheltered one, and therefore the 

 waves have no effect in weathering and dwarfing the shells such as 

 we often see on exposed coasts like the Land's End. The effect of 

 frost may be at once put aside. When the temperature falls below 

 the point at which a mollusc can assimilate its food the develop- 

 ment ceases, and a similar effect is produced by too great heat. 

 Littorina Rudis ranges as far north as Spitzbergen and Hamilton's 

 Inlet, and therefore should be able to endure severe cold. Severe 

 heat in our climate it is not likely to suffer from. The effect of 

 volume of water may be disregarded. In general the greater the 

 volume of water the greater the length of the shell. This is, of 

 course, subject to much change on account of other causes of 

 variation, such as increased or diminished quantity of food for each 

 inhabitant, and is usually so overpowered by them that its effect is 

 inappreciable. 



Increased or diminished saltness of the water requires far more 

 consideration. This undoubtedly has some effect on the form of 

 the shell, but does not, I think, produce monstrosities. There is 

 an interesting paper by Mr. Bateson,* in which he discusses some 

 forms of Cardium Edule from the Aral Sea and from Egypt. He 

 draws the conclusion that increased saltness due to evaporation 

 produces thinner shells and a higher colour, while increased fresh- 

 ness due to the influx of freshwater does not produce a thinner shell. 

 In the Cardium increased salt increases the breadth in proportion 

 to the length. He considers the change to be due to "general 

 unfavourableness of conditions," and says further " all that can be 

 stated with certainty is that shells exposed to increasingly saltwater 

 do change in a particular way, and they do so with great regularity 

 and uniformity. In the same way it has been shewn that the 



* Phil. Trans, of Roy. Soc. of Lon., vol. Icxxx. (1889), B., pp. 297-330, 



