THE MARINE FAUNA OF THE HUMBER DISTRICT. 33 



weather to leave their crabpots out until the middle of August, 

 I was able to secure man)- deep water forms. The most 

 interesting capture was the hermit crab in his accustomed 

 home, an empty whelk shell, with the upper whorls tenanted 

 by Nereis fucata and the crustacean, Ncenia tuberculosa, 

 sharing the lower. 



In addition to all these localities, in which work is 

 necessarily discontinuous, the brackish ditches of the Humber 

 district offer a wide field for investigation without any of the 

 discomforts of mud-wading, and have already provided 

 several interesting discoveries. Here we may watch the 

 gradual change as the water becomes less brackish year by 

 year, or note the rarer converse effect on the flora and fauna 

 when by some means the salt water enters a normally fresh 

 water stream. As a rule, fresh water species are killed by a 

 very slight admixture of salt water, but marine (estuarine) 

 species can endure a wide variation of salinity; and this< 

 endurance creates a feeling of astonishment that the instances 

 of migration from salt to fresh water are so few. 



In the Hedon drain, the shore crab and the plaice have 

 been caught in practically fresh water, and Tellina balthica 

 was obtained in abundance several years ago in the lower 

 reaches where the fresh water flowed over it at every low tide. 

 During the last two years, owing to the imperfect clough, 

 the drain has become more brackish, and in August, 1902, I 

 took the common shrimp (Craugou vulgaris) and Xcomysis 

 vulgaris, half a mile from the outfall, where formerly we 

 caught roach. The lower limit of the flowering rush, the 

 arrow head, and the fresh water mussel has been pushed 

 back a mile ; even Potamogeton pectinatus does not flourish 

 in the lower reaches, and the drain no longer turns red with 

 unicellular alga? in August. An instance of the converse 

 occurs on Sunk Island in a drain on the 1850 enclosure which 

 formerly entered the old North Channel by a clough (dated 

 1850) which is now blocked up. Palcemonetes variaus lives 

 with Limnea peregra and the water crowfoot in the drain, and 

 Orchestia littorea is found on its banks. P. ventrosa, Entero- 

 inorpha, and the other inhabitants of brackish water are 

 absent. 



Another instance is found at Patrington Haven, which was 

 navigable in the middle of the last century. It then opened 

 into the North Channel, a relic of the former channel round 

 Sunk Island, but owing to the' reclamation of the eastern 

 parts of Sunk Island in 1826 and 1850, it became so warped 

 up that all traffic was discontinued. Subsequently, the North 



