MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 343 



the closing of the Ubrary of the Zoological Society during the 

 month of September, I regret that I am unable to show this 

 paper to-night. For the majority of species there is a certain 

 area in which the shell exhibits its most typical form and in 

 which it is most numerous. Beyond these limits it is more apt 

 to vary and to form races and varieties. Of course where it is 

 most abundant it is more common to find monstrosities, either 

 due to injury or freaks of development, as ovate shells becoming 

 acuminate like Bucciniim imdatum var. acuminata, globose 

 shells disciform as in Helix nemoralis var. planospira of Picard, 

 in which the whorls are depressed until it resembles a Planorbis. 

 The whorls also may be partially or completely disjointed so as 

 to produce turreted, scalariform, or even ceratoid forms like a 

 ram's horn, as in the specimen of Helix aspersa in the drawer 

 before you. 



Monstrosities may also be produced by external injury, as 

 where a Unio has the valves partly fastened together by the 

 byssus of Dreissena \ and in an individual of Helix ieirestris in 

 the British Museum, which has a small stick passing through it ; 

 and in my Helix hoitensis which, when small, got into a nut- 

 shell, and not only carried it on its back during the remainder 

 of its life, but also partly utilized it in the growth of its shell. 

 At Montpellier Helix aspersa, deprived of its shell, was placed 

 in the upper parts of the shells of Paludina and lantJiina com- 

 munis, and a perfect compound shell thus formed like the speci- 

 mens exhibited. 



Johnston records the occurience of an oyster in the Firlh 

 of Forth in which the interference of three corallines growing 

 near its edges distorted it so that it resembled an ace of clubs. 

 Again, the effects of injury may be to render necessary the dis- 

 use of a mouth and the formation of a new one.— Clausilice 

 have been described furnished with as many as four mouths. 



Boring mollusca and shells which are closely applied to 

 other surfaces, such as Anomice and Cupula, also show con- 

 siderable variation due to these causes. You will see Anomice 



