178 



ECHINOIDEA. II. 



arctic fauna to proceed to Cape Cod, on 42° Lat. N., the latitude of Northern Spain. The study of the 

 o-eographical di.stribution of marine auiiuals therefore mu.st rest on the study of the physical conditions 

 of the sea. 



It is a natural thing that it is upon the whole impossible to fix the limits of the different 

 reo-ions very definitely. Most species pass over the limits, and very few species occur exclusively 

 within the limits of one region, whereas very many species are common to two or more regions. 

 There is thus generally a rather extensive area between the adjoining districts, where the species 

 peculiar to each district meet and intermingle, the fauna thus being composed of elements from the 

 two adjoining regions, without any forms peculiar to it. Such areas are e. g. the tract from the 

 Channel to Morocco on the European side and from Cape Cod to Cape Hatteras on the American 

 side of the Atlantic. — It is the same with the depth-limits of the regions. Most of the species 

 have a very extensive range in depth, several of them ranging even from quite shallow water down 

 to the great abj-sses (though it is generally easy to decide if a species is mainly littoral or abyssal) 

 The bathymetrical limits of the regions must then necessarily look somewhat arbitrary, from a 

 systematic point of view. But, on the other hand, Agassiz may thus far be fully justified in saying 

 («Challenger»-Echinoidea. p. 222) that «the divisions into littoral, continental and abyssal or oceanic are 

 not arbitrary; they represent in the present state of our knowledge of the depths of the oceans, bathy- 

 metrical lines of great physical importance. The littoral fauna extends over that shallow area of the 

 shores which is merely the extension under water of the shores themselves (to 100 or 150 fathoms); 

 the continental line represents the extent to which we may fairly assume that the lines of continents 

 have been modified, tlie limits within which probably subsidence and elevation as affecting continental 

 masses, or rather their shores, have taken place, to 450 or 500 fathoius, while the third region beyond 

 this, that which has been called abyssal or oceanic, undoubtedly represents those large areas of the 

 ocean floor which have remained unaffected through long geological periods). 



In Part I (p. 28) I have distinguished between the littoral belt, the sublittoral, archibenthal and 

 abyssal belts. The littoral is reckoned from o — ca. 50 fathoms, the sublittoral from ca. 50 — ca. 300 

 fathoms, the archibenthal from ca. 300— ca. 1500 fathoms, and depths greater than 1500 fathoms are 

 called abyssal. These divisions are certainly not so fortunate as those maintained by Agassiz, and I 

 therefore give them up and follow Agassiz, recognizing only three main bathymetrical divisions. 

 The littoral region I prefer to limit to the 100 fathoms; the next region then goes from 100 to 500 

 fathoms, viz. tlie archibenthal region ( — this name seems to mc preferable to the name «continental», 

 which has also another meaning in Zoogeography), and the depths below 500 fathoms make the abyssal 

 region. — There is, however, no reason for maintaining these depth regions for all the districts. The 

 European boreal and the Mediterranean regions have in so far the same faunistic character throughout 

 their whole bathymetrical extension that not a single species is characteristic for the greater depths 

 alone, below the 100 fathoms line (except Spatangus Rasclii, which is, however, probably only an im- 

 migrant into the European boreal region). The only difference is that the greater depths in these 

 regions are much poorer in Echinoids than the littoral regions, several species being strictly littoral, 

 as far as hitherto known. This especially holds good for the Mediterranean. — In a more detailed 

 account of each region there is reason for distinguishing between the strictly littoral, sublittoral 



