CLARK: GEOGRAPHICAL RANGE IN RECENT CRINOIDES 561 



greater value from a bathymetric than from a geographic stand- 

 point, so that, broadly speaking, our knowledge in regard to the 

 bathymetric distribution of animal types is much more detailed 

 than our knowledge of the hmits of the geographical range of 

 the same types. 



The bathymetric range is measured on a single line assumed to 

 extend perpendicularly downward from the surface to the deepest 

 part of the sea. But the points on this line are determined from 

 observations everj^^here. That is, the entire volume, or cubical 

 content, of the ocean basins furnishes data projected upon a 

 single line. For example, a certain type occurs in Alaska in 5 

 fathoms, in the Crozet Islands in 1600 fathoms, and off Green- 

 land in 300 fathoms; its bathymetric range is, therefore, from 5 

 to 1600, or 1595, fathoms. 



Thus our knowledge of the bathymetric ranges of the larger 

 groups is reasonably complete, as a result of this method of 

 reducing to terms of a single dimension records which, strictb^ 

 speaking, are taken in three dimensions. 



With the geographical ranges calculated as the sum of all the 

 areas measuring 15° on each side within which a given type occurs 

 the case is very different. Investigation has largely been localized 

 within certain restricted areas which, for some reason or other, 

 have proved to be especially interesting, or where exceptional 

 opportunities for study have been presented. Within these 

 areas there are very numerous records all of which, though mark- 

 ing different steps in, and increasing our knowledge of, the bathy- 

 metic scale, fall in the same geographical unit, and hence are the 

 geographical equivalent of onl}^ a single record. 



Intensive study of any one locality increases our knowledge of 

 the bathymetric distribution of all the endemic types, while 

 adding nothing to our knowledge of the geographical distribution 

 of the same types; similarly future investigations in many of the 

 now httle known regions of the world will greatly increase our 

 knowledge of the geographical range of many groups, at the same 

 time giving us nothing new in reference to their bathymetrical range. 



Thus it is evident that the comparison of the bathymetric 

 ranges of the families of recent crinoids, which for the most 



