SYKES: VARIATION IN RECENT MOLLUSCA. 



261 



beds the species in the lowest beds are smooth, higher up they become 

 angular and carinated, and finally they are sub-spinose. 



Sculpture, as is well known, is due to increased secretion at certain 

 points, but how and why are these points determined ? 



In this connection two very interesting papers dealing with the 

 Gastropods merit consideration. Dall ^ was led to consider the folds 

 on the columella in the Volutidae, and he pointed out that in the 

 molluscs with folds on the columella the adductor muscle was situated 

 more deeply in the shell than in those in which the folds were absent. 

 Further, he stated that when the mantle-cone was withdrawn within 

 the shell, it must wrinkle longitudinally, and the more deeply-seated 

 the adductor muscle was, the stronger would be the wrinkles, the 

 deeper wrinkles coming on the columellar side, and the more numerous, 

 smaller ones being opposite to the pillar. From this he was led to 

 consider that the folds on the columella and the lirse or denticles 

 within the mouth were formed by the repeated dragging in and out of 

 the shell-secreting surface of the mantle, the folds or lirse forming in 

 the wrinkles. This explains the folds in some forms of Cyprcea, which 

 are seen only in the adult shell, since in the young the mouth is more 

 widely open, and the mantle not so wrinkled. It would be of interest 

 to know how far the remarkable internal armature in Plectop)jlis, 

 formed only at certain intervals in the shell, is due to such wrinkling 

 of the mantle at rest periods, and whether their constancy in form is 

 due to the mantle being folded naturally in a permanently similar 

 manner, conformably to the internal organization of the animal. 



A further development of the same theory was made by Stearns ^ in 

 a paper on the remarkable series of variations of Paliidestrina protea, 

 Gould, in which he attributes spiral sculpture on the surface of the 

 shell to puckering of tlie mantle-edge, while transverse sculpture he 

 points out may be formed owing to a short period of rest when the 

 edge of the lip is receiving the bulk of the secretions. 



For further interesting details the student should consult these two 

 papers. 



There are giants and dwarfs in molluscs as in all other animals, 

 Buccinum undatum, L., for example, may measure as much as 6 J inches 

 in length, while an apparently adult specimen in my collection reaches 

 only \^ inches. Size may depend upon surroundings, and those at 

 the extreme limit of distribution of the species are often stunted in 

 growth. Neritina virginea is dwarfed in very dense water, as also in 

 fresh-water streams in Jamaica, from which the deduction has been 

 di'awn that while the animal can adapt itself the full development 

 will only be reached if the surroundings be favourable. Similarlj-, 

 Bateson^ has given an account of variation in form, etc., occurring in 

 Cardium, where the water was very saline. In Guernsey there are 

 two distinct races of Ocinebra aciculata, varying in size and colour, one 



' " Tertiary Fauna of Florida," pt. i, pp. 58 et seq. ; Amer, Nat., 1894, pp. 909-914. 



2 Proc. U.S. Mus., vol. xxiv, pp. 271-299. 



3 Phil. Trans., 1889 b, pp. 297-330. 



VOL. VI. .TINE, 1905. 19 



