Try these:
Gould, S. J., Young, N. D. & B. Kasson. 1985. The consequence of being
different: sinistral coiling in Cerion. Evolution 39: 1364-1379.
Vermeij, G. J. 1975. Evolution and distribution of left-handed and
planispiral coiling in snails. Nature 254: 419-420.
G. Thomas Watters, PhD
Curator of Molluscs
Museum of Biological Diversity
Department of Evolution, Ecology & Organismal Biology
The Ohio State University
1315 Kinnear Road
Columbus, OH 43212
v: 614-292-6170
f: 614-292-7774
Visit the Molluscs Division at:
http://www.biosci.ohio-state.edu/~molluscs/OSUM2