. The Conchologists' exchange. Mollusks. Crepidula fornicata. while one, C. plana, grows upon the inside or concave surface. '-The curvature of the surface upon which the Crepi- dulas grow has naturally produced a similar curvature in their own shells as it would other- wise be impossible for them to retain their hold. We therefore find that the three species first mentioned are convex in shape, while C. plana is concave. With this fact before us the question naturally arises : why is not C. plana a mere form of C. fornicata produced from eggs of the latter spe- cies which happened to be depos
. The Conchologists' exchange. Mollusks. Crepidula fornicata. while one, C. plana, grows upon the inside or concave surface. '-The curvature of the surface upon which the Crepi- dulas grow has naturally produced a similar curvature in their own shells as it would other- wise be impossible for them to retain their hold. We therefore find that the three species first mentioned are convex in shape, while C. plana is concave. With this fact before us the question naturally arises : why is not C. plana a mere form of C. fornicata produced from eggs of the latter spe- cies which happened to be deposited on the inside of the host shell? A series of specimens collected with their hosts, however, shows that young fornicata is sometimes found on the inner or concave surface of Natica and Fulgnr and retains all the characters of the species, being easily separated from the young of plana. The fact, however, that none but very small fornicatas are found in such positions, seems to show that the tendency to a convex form makes it impos- sible for them to retain their hold for any length of time on a concave surface. In other words, Crepidula plana, the tendency towards convex and concave shells has now become a fixed specific character. As to the original ancestry of C. plana there does not seem to me any doubt but that either this species has descended from a shell of the C. fornicata style or better perhaps that both species have developed from a free snail which formed the habit of attaching itself to other shells and whose characters have been gradually altered to suit its habits. Some of the individuals have thus adapted themselves to the convex and some to the concave surfaces of their hosts, and so eventually produced two distinct species. This theory is strengthened by studying the Crepidulas from other parts of the world, for my friend Mr. Pilsbry, tells me that on nearly every coast where a convex Crepidula is found there is also a concave species corresponding to our
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectmollusks, bookyear188
