244 



THE SEA SHORE 



to the right of it on Plate V., belong to the family Littorinidce, the 

 members of which are similar in structure and habit to Trochus, 

 but the shell is usually more depressed, and is never pearly. The 



FIG. 171. 1. Trochus zizyphinus. 2. UNDER SIDE OF SHELL. 

 3. Trochus magnus. 4. Adeorbis subcarinatus 



shell of the Periwinkle is thick, having but few whorls, and is not 

 umbilicated ; and the lingual ribbon, which is coiled up on the 

 gullet, contains no less than about five hundred rows of teeth ; but 

 only a little more than twenty of these rows are in action at any 

 one time, the remainder being a reserve stock 

 to come into active service as the ribbon grows 

 forward. In the genus Lacuna there is a narrow 

 umbilicus, and the aperture of the shell is semi- 

 lunar in form ; and the species of Bissoa are 

 very small, with white or horny shells, much 

 more pointed and having more whorls than 

 those of the Littorina. 

 FIG. lT2.2iissoa Our next illustration shows three shells of 

 labiosa AND the family Turritellidas, so named from the 

 acui a pa resemblance of the shells to a tower or spire. 

 The form indeed is so characteristic that they 

 can hardly be mistaken. It will be seen that Turritella communis 

 is striated spirally, while the surface of Scalaria communis (Plate V.) 



