CHAP, i.] INTRODUCTION. 15 



changes which these species have undergone since the 

 raising of the Isthmus of Panama and the isolation 

 of thetwoTaume?" 1 



Edward Forbes distinguished round all seaboards 

 four very marked zones of depth, each characterized 

 by a distinct group of organisms. The first of these 

 is the littoral zone, the space between tide-marks, 

 distinguished by the abundance of sea-weeds, on 

 the European shores of the genera Lichina, Fucus, 

 Enteromorpha, Polysiphonia, and Laurencia, which 

 severally predominate at different heights in the 

 zone, and subdivide it into subordinate belts like 

 a softly-coloured riband border. This band is under 

 very special circumstances, for its inhabitants are 

 periodically exposed to the air, to the direct rays 

 of the sun, and to all the extremes of the climate 

 of the land. Animal species are not very numerous 

 in the littoral zone, but individuals are abundant. 

 The distribution of many of the littoral species is 

 very wide, and some of them are nearly cosmopolitan. 

 Many are vegetable feeders. Some characteristic 

 genera on the coast of Europe are Gammarus, 

 Talitrus, and Balanm among Crustacea, and Lit- 

 torina, Patella, Purpura, and Mytilus among Mol- 

 lusca, with, under stones and in rock-pools, many 

 stragglers from the next zone. 



The Laminarian zone extends from low-water mark 

 to a depth of about fifteen fathoms. This is specially 



1 Preliminary Report on the Echini and Starfishes dredged in Deep 

 Water between Cuba and the Florida Heef, by L. F. de Pourtales, 

 Assistant U.S. Coast Survey; prepared by Alexander Agassiz. 

 Communicated by Professor B. Peirce, Superintendent U.S. Coast 

 Survey, to the Bulletin of the Museum of Comparative Zoology, 

 Cambridge, Mass., 1869. 



