CLARK: DISTRIBUTION OF ECHINODKRMS 625 



The types inhabiting it, that is to say, occurring only on the 

 extreme periphery of the Indo- Pacific faunal area (of which the 

 Mediterranean Sea was at one time a part), may be considered, 

 therefore, as reHcs of a previous fauna at one time characteristic 

 of the central Indo-Malayan region, from which they have 

 now been extirpated through the competition of younger and more 

 efficient types. 



The same facts are brought out equally well in many other 

 groups of marine animals, and are also reflected in a modified 

 way in the terrestrial faunas. 



In the faunas of the colder seas all intergradations are found 

 between types which are quite unique and types differing little or 

 not at all from others in the Indo-Malayan region, and this inter- 

 gradation is complete enough so that we are justified in con- 

 sidering the fauna of the colder waters as similarly ultimately 

 derived from the (past or present) fauna of the East Indian 

 region largely through the intermediary of deep water forms. 

 Some of these genera of the colder waters, as Astriclypeus and 

 Glyptocidaris, are extraordinarily restricted in their distribution 

 and rare, while others are abundant and widely spread. 



