If one is interested in the Littorina I would recommend getting a copy of
Systematics and Evolution of Littorina by David G. Reid published in 1996.
It is not cheap, about $160 or so, but it excellent in tracing the
synomonies of the various valid species and has great illustrations.
 
Charlie
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Charlie Sturm, Jr
Clinical Instructor - Family Medicine
  University of Pittsburgh School of Medicine
Research Associate - Section of Invertebrate Zoology
  Carnegie Museum of Natural History
 
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