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THE AMERICAN NATURALIST [Vol. XLVII 



sented by the total number of specimens obtained. A 

 more accurate and justifiable presentation is to express 

 the total number of specimens obtained from each of the 

 above depths in terms of the average number per unit of 

 time consumed in hauling. The following table reveals 

 this relative abundance of the three species as thus deter- 

 mined : 



TABLE IV 



Eelative Abundance or Average Number op Specimens Obtained per 20 



It is evident that this table brings into still more strik- 

 ing relief the fact that S. bipunctata is most abundant 

 between the surface and 25 fathoms, from where it de- 

 creases in abundance as the depth increases, while S. 

 serratodentata increases from a minimum between the 

 surface and 25 fathoms to a maximum between 150 and 

 250 fathoms, and 8. lyra increases from a minimum near 

 the surface to a maximum in the deepest water (250 to 

 350 fathoms). While it is very improbable, owing to 

 variations in many environmental conditions affecting the 

 abundance of the three species in the various depths, that 

 subsequent collecting would ever result in exactly the 

 same averages as given above — it is just as improbable 

 that, if the hauls were distributed in approximately the 

 same manner, with regard to such environmental condi- 

 tions, as those from which the above data were derived, 

 we should find the relative abundance much altered. Con- 

 sequently it is no exaggeration to say that each of these 

 three species has its own definite and specific manner of 

 vertical distribution just as truly as each has its own 



1 As relative abundance is independent of the particular unit of time 

 selected for standardizing the data, a unit of 20 hours has been used instead 



