198 MONSTROSITIES OF LITTORINA RUDIS. 



form. The only colony I am aware of in which the presence of 

 weed has not been noticed is that of the Littorina Littorea at 

 Belfast. For the purposes of my argument it is hardly necessary 

 that it should have been present since these shells are hardly 

 monstrous. I might perhaps mention while suggesting that the 

 tufts of weed cause the variation in form that Koffiaen* unsuccess- 

 fully endeavoured to produce scalariform varieties by applying 

 gypsum to the aperture of living specimens of Helix during their 

 growth. The insertion of a piece of stick or some such cause has 

 frequently, however, been the cause of a monstrosity. The fact 

 that no adult forms have been found, I incline to think, supports 

 the theory that they are due to weed ; as if due to some condition, 

 such as addition of freshwater, they should be as healthy and as 

 likely to come to maturity as the normal ones. Weed would also 

 be more likely to affect the weakly specimens, as the stronger ones 

 would push it aside more readily. Also, might not the curious 

 form that they are compelled to take in some way unfit them for 

 the surrounding conditions and thus cause their extinction 1 



To sum up shortly then. I think that the cause should not be 

 sought in the animal since the earlier whorls are always regular. 

 Hybridism is not possible since they exhibit only the characters of 

 L. Rudis. Variation in temperature does not produce monstrosities ; 

 neither does absence of salt, unless, possibly, a total lack. We are 

 then left with weed, which seems to be found with all monstrosities 

 in colonies, and which I incline to believe the probable, as it 

 certainly is the possible, cause. 



* Ann. Mai. Belg., vol. iii., pp. Ixxxii. Ixxxiv. 



