28 
CALIFORNIA SEA-SIIELLS. 
Thera are several other species of Bittium, most 
of which have ribs. 
Fig. 4, PL VI, represents a Scalaria or Stair- 
case Shell, probably of the species named crebri- 
costata , “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 , did., 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 be 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, PL 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- 
