Responding to my last post, snail maven Aydin Örstan thought the third of the terrestrial snails harboring on the marine snail shell in my backyard was Vallonia costata. If so — and it looks like it to this mollusk amateur — that would make for five different species of snails found in my concrete slab of a Brooklyn backyard so far.
This snail is tiny, 2mm across, and posed here on FDR’s eye on a dime. My antique snail book, Shells from Cape Cod to Cape May with Special Reference to the New York City Area (Dover, 1971, reprinting a 1961 original) by Jacobson and Emerson, notes that this Eurasian species wasn’t reported in the NYC area until “recently” when some were found in the Bronx, in a colony subsequently destroyed by construction. Jacobson and Emerson inform me that the ridge-like axial ribs on this shell are also known as costae.The hollow at the center of the whorl is called an umbilicus.