1234 BULLETIN OF THE BUREAU OF FISHERIES. 



leaving only 37 per cent which, so far as reported, have not transcended this 

 barrier. Doubtless more complete information will reduce the latter figure. 

 As has already been pointed out, any locality where extensive collecting has 

 been done is sure to figure as the reputed limit of distribution — whether northern 

 or southern — for many species. It is significant, therefore, that only 40 of the 

 species under consideration (20 per cent) have not yet been recorded from 

 points south of Woods Hole." Comparing this figure with the 37 per cent 

 which are not known to occur north of Cape Cod, it may be that we have some 

 measure of the real effectiveness of the last as a barrier to distribution. 



It must be conceded at once that it is impossible to form a just estimate 

 of the geographical range of a species from any mere statement, however correct 

 in itself, of the extreme limits of its distribution. The bathymetric range and 

 other factors of its habitat at various latitudes must be taken into consideration. 

 It is obvious, likewise, that the same importance must not be attributed to the 

 isolated and occasional occurrence of a given species as to its occurrence at 

 points where it is widespread and abundant. But in most of the published 

 tables which are available for consultation no distinction is made between 

 the two. 



Crude in the extreme, therefore, as any such computations must be, the 

 conclusions seem to be fairly well grounded (i) that Cape Cod does have an 

 appreciable influence as a barrier to distribution, and (2) that the southern 

 types preponderate considerably over the northern ones in our Woods Hole 

 fauna, or at least such of it as is accessible to the dredge. These generalizations 

 may not be true of each individual group (e. g., coelenterates and amphipods) ; 

 and in general it must be remembered that a considerable minority of northern 

 forms are included in our local fauna, while more than 60 per cent of our species 

 are known to occur north of Cape Cod. On the other hand, it is well to add 

 that our local fish fauna, which is but sparingly represented in our dredging 

 records, and consequently plays little part in the foregoing tabulation, is over- 

 whelmingly southern, 75 per cent being southward-ranging in the foregoing sense 

 of the term, while nearly 50 per cent of the total number of recorded species are 

 such as are reputed to find in Cape Cod their northern limit of distribution. 

 And, lastly, we must bear in mind that we are here dealing only with the benthos 

 of the region, the plankton, as well as the littoral fauna, being left out of con- 

 sideration. ' 



Turning now to another phase of our results, the comparative distributions 

 of different species of the same genus are presented by us in a large number of 

 cases (e. g., Pecten, Astcrias, Crepidula, Pagurus, fig. 16, 17, 20-27). In some 

 cases, two such species have a practically coincident distribution, as regards both 



o More strictly, south of Vineyard Sound and Buzzards Bay. 



b Of the total number of i ,600 (±) species of animals recorded for this region, only 500 (-|-) have been 

 taken during our dredging, and of these less than half have been employed in the above computations. 



