132 CLARK: BATHYMETRICAL DISTRIBUTION OF CRINOIDS 



In this connection it must be borne in mind that the crinoids 

 are confined to clear water of a comparatively slight range in 

 salinity and in composition, and with a very low maximum of 

 silt. They therefore are unable to exist in many of the locali- 

 ties in which other ancient types, such for example as Artemia, 

 Xiphosura and Lingula, find a safe refuge from more efficient 

 competitors, with the result that each succeeding type is neces- 

 sarily brought into direct and intimate contact with the greater 

 part, or even all, of its predecessors. 



A comparison between the average depth inhabited by each 

 group (excluding the Stephanometridse, Tropiometridae and 

 Holopodidae, monotypic, and the Apiocrinidse and Phrynocrinidae, 

 imperfectly understood) calculated as the average of the mean 

 depth of all of the component genera (or higher groups), and the 

 mean depth of each group as represented by the mean between 

 its two extremes, expressed as a per cent obtained by dividing 

 the former by the latter, gives the following figures: 



per cent per cent 



Antedonidae 28 Zygometridifi 66 



Capillasterinae 37 Charitometridse 69 



Zenometrinse 39 Ptilometrinse 72 



Thalassometrinse 42 Thysanomet rinse 72 



Colobometridse 46 Himerometridae 73 



Heliometrinse 48 Perometrina? 82 



Antedoninse 61 Bourgueticrinidae 82 



Calometridse 61 Pentametrocrinidse 87 



Mariametridse 62 Bathymetrinse 90 



Comactiniinse 62 Plicatocrinidse 97 



Comasterinse 64 Atelecrinidse 125 



Average 66 per cent 



Contrasting older and less speciahzed with more recent and 

 more specialized groups, we have: 



More specialized More primitive 



per cent per cent 



Articulata 41 Inadunata 97 



Pentacrinitidse 28 Bourgueticrinidse 82 



Comatulida 27 Pentacrinitida 63 



Oligophreata 15 Macrophreata 46 



The groups including stalked species show the following 

 relationship : 



