370 MARINE INVERTEBRATES 
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 ee 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 
Crucibulum 
siriatum trom & Cup-Shaped appendage attached by one side to the 
shinai inner margin of the shell. This latter feature at once 
determines the genus. 
C. striatum. This species has radiating riblets, cut by 
circular lines of growth. No dimension would quite reach 
Ti an inch. Itis a common shell on the Atlantic coast, and 
Crucibutum will be found adhering to stones and other shells, but it 
sie a 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 LITTORINIDE 
Genus Littorina 
Littorinais 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 lit- 
toral 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 abun. 
dant evidence of having been at some past time aquatic or ma 
rine species. These changes in nature are constantly going on 
and the gradual substitution of a lung fora gill is no very start 
ling metamorphosis. 
