28 CALIFORNIA SEA-SHELLS. 



There are several other species of Bittium, mcst 

 of which have ribs. 



Fig. 4, PL VI, represents a Scalaria or Stair- 

 case Shell, probably of tho species named crebri- 

 eostata, "close-ribbed," by P. P. Carpenter. It is 

 a beautiful, pure white shell, commonly smaller 

 than the figure, marked with about 15 sharp 

 ribs, which form a sort of crown at the suture. 



Opalia borealis, Gld., Fig. 5, PL VI, is about 

 an inch long, white, strong, with fewer and 

 blunter ribs than Scalaria. It also differs from 

 it by not having a continuous peristome. 



Upon the rocks, from the region of low tide 

 to the surface of cliffs washed by the highest 

 spray, may bo found great numbers of little, 

 dark colored shells, about the size of peas. They 

 belong to the genus Littorina, which obviously 

 means Shore-shell. Our English cousins call 

 them Periwinkles. The aperture to these shells 

 is entire, and the operculum is thin, horny, and 

 few-whorled. We have two common species, 

 both of which are nearly black, though many 

 specimens may be found which are spotted, strip- 

 ed, and even almost white. 



Littorina planaxis, Nutt., Fig. 6, PI. VI, has 

 a short spire, round body whorl, and sharp out- 

 er lip ; it is distinguished however, as its name 

 indicates, by its flattened and scooped columella. 

 Its length is from three-fourths of an inch down- 

 ward ; the interior of the mouth is brown. The 

 other species, Littorina scutulata, Gld., Fig. 7, 

 PL VI, is generally smaller and more pointed. 

 The spire is as long as the aperture, the colum- 

 ella not excavated, and the interior of the aper- 



