20 BIOLOGICAL SOCIETY OF WASHINGTON. 



dredged. We are yet ignorant as to whether the abyssal and 

 arehibenthal faunae shade gradually into one another, as seems 

 most probable ; or whether there is any line of depth, coinci- 

 dent with a temperature limit, which really fixes a boundary 

 for the abyssal fauna. 



Then, again, the difficulty and time involved in a cast of 

 over one thousand fathoms are so much greater than if it were 

 made in half that depth, that it is impossible to say what pro- 

 portion of the disparity in population between the Arehibenthal 

 and Abyssal areas, which dredgings seem to indicate, is due to 

 the fact that the latter have been far less efficiently explored. 

 The only thing of which I feel confident is that it is yet too 

 early for extensive numerical comparisons or deductions based 

 wholly on statistics. I shall therefore content myself here 

 with a very modest table, which is intended to illustrate the 

 peculiarities of the collection made during the past ten years 

 by the U. S. Steamer Blake and recently reported on by me. 



It is probable that it is a fair example of abyssal mollusk 

 faunas, but this cannot be claimed with certainty. 



The first table shows the general numerical results for the 

 Blake collection, assorted among the great systematic groups 

 and the three bathymetric zones or areas. The second table 

 shows the proportion to the whole population of the abyssal 

 region borne by those genera which exceed a single species. 

 The result here shown is that less than thirty -seven per cent, of 

 the genera comprise more than sixty-eight percent, of the spe- 

 cies ; and out of these, three families, Pleurotomidee \ Ledidu\ 

 Dentaliidce furnish nearly twenty-eight per cent, of the species 

 of the abyssal fauna collected by the Blake. 



