418 



THE AMERICAN NATURALIST [Vol. LIII 



The nature of the background provided on this rock 

 surface is by no means constant in different localities. 

 Commonly, as in the case of rocks in sheltered places, 

 completely submerged at high water, or on lee-shores 

 generally, the limestone is covered by a feltwork of 

 matted Enteromorpha and associated plants, sun- 

 bleached to an olivaceous yellow tint. This is the usual 

 condition within the sounds (Great Sound, Castle Har- 

 bor), and along the shores of bays protected from the 

 surf. In places not so sheltered the rock may be colored 

 by minute green algae, or by the growth of calcareous 

 green or reddish algae, with which, as in the former case, 

 many serpulas are intermingled; barnacles {Tetradita 

 poros(f) also sometimes extend into the OhcJddlum zone, 

 but the })i('Viiiliiio- line, and the color of any particular 

 si)(it (if -mall ;ii-.-a, is largely determined by algae. 



(hich '/fliiiiii also occurs, though more sparingly, on sur- 

 faces of a still different type. In som(> places the layer 

 of red earth (clay) underlying superficial jeoliaii lime- 

 stone is exposed at the water's edge, usually iu a small 

 cove containing loose rocks left bare by Ili(> tide. This 

 earth provides a muddy la\-ei- of icddisli l)i-own, over 

 which Onchidia are occasioiiallx round lo he creeping. 

 From such tiny coves the <lin'('t ia\s ot' the sun are 

 largely excluded, so that the In -lim- ■ >ui laees of the 

 stones are overgrown by alga', ureeu or red-brown, un- 

 bleached; on these surfaces also (h/rhi/lnnn may be 

 found. 



The zone inhabited by Onchidium is also that in which 

 great nundxM-s of the common Modiolufi occur. These 

 muss(>ls, as already stated, almost invariably line the ap- 

 proaches to th<' Onchidium nest, and grow within the 

 eiii-ii'iL; i.;i--,m-.'\vay itself. Kxcept where the mussels 

 ha\e -.Hied thickly in more or less horizontal fissures 

 betwtMMi the wind formed strata of the limeston(\ they 



