CHAP. 1. ] INTRODUCTION. 15) 
changes which these species have undergone since the 
raising of the Isthmus of Panama and the isolation 
of the two Faune ?’’? 
Edward Forbes distinguished round all seaboards 
four very marked zones of depth, each characterised 
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, Polysiphona, 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 Balanus among Crustacea, and Lit- 
torina, Patella, Purpura, and Mytilus among Mol- 
lusea, 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 
* Preliminary Report on the Echini and Starfishes dredged in Deep 
Water between Cuba and the Florida Reef, 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. 
