no. 1808. RECENT AFRICAN CRINOIDS— CLARK. 11 



remaining of the zoogeographic progression on the tropical west Amer- 

 ican coast. Moreover, the geology of the country between North 

 and South America shows that there has been a fairly permanent 

 land barrier at all times between the Caribbean and the Pacific ; such 

 channels as have intercommunicated between them could never have 

 been deep, as none of the characteristic animals occurring below the 

 sublittoral zone on the Pacific side appear to have passed through to 

 the Caribbean. No crinoids ever passed through in either direction, 

 and there is not even a subfamily or a genus common to both coasts 

 of Central America; but crinoids are almost entirely sessile — much 

 more nearly so than any other marine organisms — and many barriers 

 therefore operate against their dispersal which are ineffective against 

 other animals, including the other echinoderms. 1 



The crinoids, therefore, move very slowly into new territory for 

 this reason and also because of their slight adaptability to changes 

 of salinity and especially of temperature, and the fact that a few of 

 the littoral echinoids, ophiuroids, and asteroids have, during the 

 periods when communication was established between the Caribbean 

 and the Pacific, made their way from the former into the latter, does 

 not in any way disprove the hypothesis that there has at all times 

 been an insuperable barrier to the dispersal of the crinoids. 



Thus it seems necessary to assume that the West Indian crinoid 

 fauna reached the Caribbean basin from the eastward, from the 

 northwest coast of Africa, having previously reached that district 

 from the southward or southeastward, not from the eastward or 

 northeastward. 



According to this, the northwest African intermediate fauna 

 (including that of Europe south of the Bay of Biscay) is zodgeo- 

 graphically more important than the West Indian, though at present 

 much less known; the two, so far as can be judged, are practically 

 the same, so that together they may be considered as making up a 

 South European- Northwest African- Antillean faunal division of the 

 intermediate region, which falls naturally into two subdivisions, (1) 

 an Afro-European and (2) an Antillean. 



It is a very curious fact that almost without a single exception the 

 genera and species of this faunal division inhabit deeper — often much 

 deeper — water than the genera and species from which they have 

 been derived in the East Indian seas, and their habitat in the east is 

 deeper again than their habitat in the west. It would appear that 

 this faunal division has taken from the great parent Indo-Pacific- 

 Japanese region only such genera as are most plastic and can best 

 adapt themselves to changing conditions; these genera have been 

 able to survive, but have become differentiated from the parent stock, 

 while all the other less plastic genera, which we must assume were 



i Vid. Medd. fra den naturhist. Forening i K0benhavn, 1909, pp. 117-133. 



