On the Pacific coast the following species are found: ^C. adunca^, with a strongly recurved apex, and less than one inch long; ^C. rugosa^, rough, brown, with the apex lying on the edge of the shell, and one inch in length; ^C. navicelloides^, almost identical with the east-coast /C. plana/. /C. aculeata/ also appears. GENUS ^Crucibulum^ /Crucibulum/ has a peculiar rounded shield-like form, with a very small apex on one side. Within there is a cup-shaped appendage attached by one side to the inner margin of the shell. This latter feature at once determines the genus. [Illustration: /Crucibulum striatum/, from above.] [Illustration: /Crucibulum striatum/, from below.] ^C. striatum.^ This species has radiating riblets, cut by circular lines of growth. No dimension would quite reach an inch. It is a common shell on the Atlantic coast, and will be found adhering to stones and other shells, but it is not, strictly speaking, a littoral species. ^C. spinosum.^ The shell exhibits a strong tendency to spinous processes on its back. Found along the southern part of the California shore. FAMILY ~LITTORINIDÆ~ GENUS ^Littorina^ /Littorina/ is probably the most characteristic genus of Northern littoral regions. Together with some of its allied genera it is also, probably, equally characteristic of various tropical littoral faunas all over the world. The family comprises strictly between-the-tides genera and species. Indeed, it is suspected that some species of /Littorina/ are making very fair progress toward a terrestrial condition, for they actually live above high-tide mark,—even in the branches of overhanging trees,—and must certainly pass days at a time out of their natural element. That such a transformation is possible need not for a moment be doubted, for there are many land mollusks to-day that give abundant evidence of having been at some past time aquatic or marine species. These changes in nature are constantly going on, and the gradual substitution of a lung for a gill is no very startling metamorphosis. [pg371]