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 apex lying on the edge of the shell, and one 

 inch in length; C. navicelloides, almost identical with the east-coast 

 C. plana. C. acideata also appears. 



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. 



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 

 striatum, from | g no ^ s t r i c tly 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 LTTTORINIDJE 

 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 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. 



