Manual of Conchology, Structural and Systematic: With Illustrations of the Species; Acmædæ, Lepetidæ, Patellidæ, Titiscaniidæ. E-book. Formato PDF George W. Tryon   -  Forgotten Books, 2017  - 

The shells are excessively variable, as is usually the casein seden tary mollusks. The author has examined very large suites of specimens, including nearly every species and variety described from the waters of North and South America, both east and west, and of Japan, Polynesia and Europe. The Australian and New Zealand forms are known to me by fewer specimens, and a number of the species of those regions I have not seen.