454 BULLETIN OF THE 



SUMMARY. 



The attempt to prepare a summary of bathymetrical data for the deep-sea 

 fauna of any region yet investigated, is most unsatisfactory in its outcome 

 from the paucity of data. Most of the species of any collection are represented 

 by the shells alone, which may have been — as millions are daily — disgorged 

 by fishes, and never have lived at the depth from which they were dredged. 

 We are yet ignorant as to whether the abyssal and archibenthal fauna) shade 

 gradually into one another, as seems most probable; or whether there is any 

 line of depth, coincident with a temperature limit, which really fixes a boun- 

 dary 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 proportion of the disparity in population between the 

 archibenthal and abyssal areas, which dredgings seem to indicate, is due to the 

 fact that the latter have been far less efiiciently explored. The only thing of 

 which I feel confident is that it is yet too early for extensive numerical compari- 

 sons or deductions based only on statistics. I shall therefore content myself 

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

 of the Blake collection, expressly disclaiming any intention of applying the 

 results to the deep-sea population at large, except " with all reserves." 



In a general way, I may observe that the results sustain the biographic gen- 

 eralizations of the Introduction to Part I. There is nothing in that summary 

 of the conditions of abyssal life which seems, so far, in need of serious modifica- 

 tion. On the other hand, several of the hypotheses there advanced have received 

 substantial support from subsequent investigation. 



In that preface the littoral zone or area was defined as that part of the sea 

 bordering on the land not too deep for the existence of marine vegetation; in a 

 word, about the area included between the dry land and the hundred-fathom 

 line. Thence to about one thousand fathoms extends the archibenthal area, 

 beyond which we find the abyssal region. 



The Tables relate only to the species which appear in antique type in the 

 text, thus eliminating most of the forms introduced for purposes of illustration. 

 I have been assisted in making the enumeration by Mr. Gilbert D. Harris 

 of the United States Geological Survey. The first Table shows the general 

 numerical results for the Blake collection, assorted among the great systematic 

 groups and the three bathy metric 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 per cent of 

 the species; and out of these, three families, Pleurotomidce, Ledidce, and Den- 



