298 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



ever, six times in the surface net worked with an otter- 

 board, four of these being at Stat. I. and two at Stat. III. 

 In the other surface nets it occurs on three days at Stat. I. 

 and on three days at Stat. III. The greatest number is 

 500 in the Nansen net hauled from 60 faths. to the surface 

 on April 8th, in mid-channel ; but fairly large numbers 

 were also obtained sometimes on the surface in the bay, 

 e.g., 200 on February 24th, 300 on April 27th, 200 on 

 August 23rd, and 400 on December 21st. 



Consequently, having once appeared in the district 

 in '07, this species seems to have remained during the last 

 two years, and, with the exception of the great haul 

 obtained on its first appearance (September 12th, 1907, 

 2,000 in net at 10 faths. and 2,500 in net at 20 faths.), the 

 numbers during '09 seem to be on the whole larger than 

 they were in '07 and 08. 



The alternative to this being a case of an invasion 

 by a more northern species is that it is merely one of the 

 smaller and rarer forms which had hitherto escaped 

 notice. It is only 0*7 mm. in length, and was only made 

 known to science in '03; and since then it has been 

 found, by careful and critical investigation, in several 

 distant localities. Previous to our recent " intensive 

 study," it may have passed through the meshes of the 

 coarser nets generally used, or may have remained 

 uncaptured through its rarity, or if captured may not 

 have been distinguished from other small forms. There 

 are various others of the smaller and rarer forms that we 

 now get more frequently, first because of the more perfect 

 collecting, and secondly because of the more exhaustive 

 examination of the material collected. 



We put this forward, then, as a possible alternative 

 explanation, not merely in this case, but in that also of 

 other rarer species which seem to make their appearance 



