192 MONSTROSITIES OF LITTORINA RUDIS. 



to become monstrous sooner than descendants of an ancestor not so 

 affected. Clessin * states that in certain species of Planorbis of 

 which he found scalariform specimens there is no transmission of 

 these deformities, but that each is caused by obstacles or external 

 agencies. He states that scalariform specimens move more easily 

 through weed than the ordinary ones. The reversal of the twist 

 of the spire stands on a different footing and must be excepted 

 from the general remarks on monstrosities. In such cases the 

 embryonic whorls are reversed, and such a variation is generally 

 inherited by the majority of the descendants. A case has been 

 noticed by Miss Hele, in which sinistral Helix Aspersa had dextral 

 young, thus reverting to the normal form. 



The specimens which I am about to discuss belong to the species 

 Littorina Rudis, and were found in the Fleet Backwater, near 

 Langton Herring, where there is a small amount of fresh water 

 mingled with the salt. They are picked out from an unsorted mass 

 of broken shell and drift which the waves cast up on the shore. 

 At present I have been unable to find any live specimens of the 

 monstrosities, though normal ones are common alive, and those 

 figured therefore are only dead shells. The figures of these shells 

 amply exhibit their curious forms, but on those represented by 

 figs. 1, 2, 3, 4, and 12 a word or two may be useful. Fig. 12 is a 

 shell of much thicker substance than the others and of normal 

 form, found near the mouth of the Fleet, where the saltwater has 

 practically no fresh intermingled with it, and where the rapid 

 current prevents the growth of any weed. Fig. 4, unlike all the 

 others, has the tube of the whorl uneven. In this specimen the 

 tube increases and decreases in diameter alternately, thus producing 

 an undulating or wavy surface. Figs. 2 and 3 are by far the most 

 interesting ; their appearance until a close examination is made 

 being that of sinistral specimens. In fig. 2 the spire is actually in 

 the mouth of the shell. Their curious appearance is produced as 

 follows : At first the specimen is regular, then the tube of the 

 shell is gradually bent upwards, passing over or close by the spire, 

 * Malacozoologische Blatter, vol. xx., pp. 68-83. 



