OrLivER.— Marine Littoral Plant and Animal Communities. 509 
the lumen behind the last occupied whorl. Erosion is prevented in those 
species which can cover the shell with the mantle (Ancilla) or those 
possessing a periostracum (Cymatium costatum, Calliostoma) 
Sea-water is not appreciably acid, but fresh water always contains 
dissolved carbon-dioxide, and where its influence is felt along the sea- 
о 
of the shells of mollusca are more ог less eroded. Monodonta corrosa, Comi- 
nella lurida, Cerithidea bicarinata, and many others might be mentioned. 
associations. Molluscs exposed between tide-marks, and therefore to the in- 
the exception of a narrow strip along the margin of the aperture, has 
disappeared. (Plate 42, fig. 2.) All the shells also have a rusty-brown 
deposit on their exposed surfaces. 
Eparnmic FACTORS. 
Rock. 
(a.) Attachment of Plants.—The larger algae are attached by disc-shaped 
ti 
animals, which is usually the case. In Macrocystis pyrifera and стек 
variegata it consists of а mass of stems, terete and forming a dense nitur 
in Macrocystis (Plate 42, fig. 7), but in Lessonia taking the form o 
a plug-like byssus. ; : 
Mined animals not possessing calcareous shells, such as tunicates, are 
attached by their leathery tests, the base of attachment being always 
