Snail shells are said to be “scalariform” when anomalous growth causes the whorls of the spire to more laxly coiled and the spire, thus, appear longer than typical. The protoconch and early teleoconch whorls of this shell are missing or damaged which may suggest a cause for this deformation. A normal shell of Cepaea nemoralis is much more nearly globular.
A nice photograph of another scalariform C. nemoralis appears in this website: http://molluscs.at/gastropoda/index.html?/gastropoda/morphology/scalariform.html
A normal C. nemoralis is shown below.
Crepidula convexa (Say, 1824) is a northwest Atlantic slipper-snail long known to be introduced in San Francisco Bay. This marine snail species arrived on the West Coast by accident, having been transported with Eastern Oysters, Crassostrea virginica (Gmelin, 1791) (Hanna 1966). I found this species for the first time in Boundary Bay, British Columbia, in 1984, southern Strait of Georgia, British Columbia, although its first appearance in BC must have been many years prior when Eastern Oysters were being “planted” in the Bay for the commercial oyster industry.
The native range is Atlantic Canada to Georgia (Colin 2002).
Crepidula convexa is polymorphic: the shell varies considerably in colour. Many shells are creamy white or light brown with darker reddish brown spots and streaks. Others are solid purplish brown or grayish brown with darker spots that are not always clearly evident. Shell form varies as well, depending upon the substrate to which the animal was attached. Larger, oval shells (such as shown here) come from stones and other relatively flat surfaces. Narrowed, higher examples live on snail shells. In Boundary Bay living Batillaria atramentosa (G.B. Sowerby II, 1855) are often infested with C. convexa. Diminutive individuals live on eelgrass (Zostera marina).
- Collin R (2002) Another last word on Crepidula convexa with a description of U. ustulatulina n. sp. (Gastropoda: Calyptraeidae) from the Gulf of Mexico and southern Florida. Bulletin of Marine Science 70: 177–184.
- Hanna GD (1966) Introduced mollusks of western North America. Occassional Papers of the California Academy of Sciences (Series 4) 48: 1–108, pls 1–4.