A. H. CLARK : THE CRTNOIDS OF THE INDIAN OCEAN. 15 



fauna shows exactly the same features of distribution as does the Littoral, though 

 we must enlarge tlie area of maximum intensity so as to make the apices of the 

 triangle Ceylon, the Kermadec Islands, and southern Japan, and magnify the 

 ranges of the component- genera in the same proportion. Whereas southern 

 Japan, Fiji, northern Australia, and the east coast of Africa mark roughly the 

 attenuated outer limits or fringe of the East Indian Littoral fauna, for the 

 Intermediate we must enlarge these boundaries to include the western Aleutian 

 Islands, the Hawaiian Islands, the Galapagos Islands, the Kermadec Islands and. 

 properly speaking, on the west the West Indies. 



The Oceanic fauna is but an exaggerated derivative from the Intermediate, 

 and can no more be definitely distinguished from the Intermediate than the 

 Intermediate can from the Littoral. Its chief character is given by species 

 of Bathycrinus (Bourgueticrinidse), Balhymetra (Antedonida?, Bathymetrinae), 

 and Thalassometra, and by the family Pentametrocrinidse. Species of most of 

 these Oceanic groups occur everywhere in very deep water, except in enclosed 

 cold and stagnant basins, but each of the groups include species living in 

 Intermediate, or even quite Littoral depths. 



Although Baihymetra is found in the Pacific at 2,900 fathoms it does not 

 occur at all in the north Atlantic, while Thalassometra does not occur at all on 

 the American side of the Atlantic, and only south of the Bay of Biscay on the 

 European side. 



Whereas the large littoral crinoids are confined to the central East Indian 

 region and the representatives of the groups including them become smaller as 

 one moves away from this area, the crinoids of the Intermediate zone, though 

 they decrease gradually in size to the westward, increase to the eastward and 

 northward so that the largest species are found in northern Japan, the Hawaiian, 

 or the Galapagos Islands. 



There is one zoological principle well brought out by the crinoids of the East 

 Indian region which I cannot remember to have seen stated anywhere, though 

 it is equally well shown in many groups, both terrestrial and aquatic, and that 

 is, that in all natural genera which are adequately known, and sufficiently well 

 represented in the present fauna, there exists typically a single species which 

 covers the entire range inhabited by all the other species of the genus collectively. 

 This species is always the most variable, individually, of all contained within the 

 genus and, if the species of the genus be arranged according to the development 

 of the specific characters in them, this species typically falls midway between 

 the two extremes. In each family also there is typically to be found a genus 

 which in every way corresponds to this species. 



It is noteworthy that in the case of most, if not all, species, individuals from 

 the outskirts of its geographical range are the most variable, the coefficient of 

 variability decreasing toward the centre ; and in each natural genus the species 

 inhabiting the extremes, bathymetrical or geographical, of the genus as a whole 

 exhibit the same peculiarity. 



