igio. CoLGAX. — The Adaptability of Littoral Molhisca. 129 



instance, however, did an}^ of the many specimens tested sur- 

 vive even a twenty-four hours' immersion. As for Hydrobia, 

 there is nothing in its habits to render probable the capacity 

 which a long series of observations proved it to possess of 

 enduring j)rotracted exposure in dry air and immersion in 

 strong brine. 



Coming to the second branch of the inquiry, it would seem 

 a priori \X\2iX. taking each species by itself those individuals 

 collected from stations at or above high-water mark should 

 shew a greater tolerance of exposure to air than the indivi- 

 duals of the same species collected from lower levels where 

 the}' are dail}' submerged in sea water for lengthened periods. 

 And a priori one might expect, too, that in each species the 

 high-level individuals, accustomed as they are to exposure to 

 dew and rainfall, would show a greater tolerance of fresh 

 water and a less tolerance of water of high salinitj' than the 

 low-level individuals which pass the greater part of their lives 

 immersed in sea water. 



A series of observations made in Januar}^ last showed con- 

 clusively that these expectations were fully justified. Two 

 species were selected for this purpose, Purpura lapillus and 

 Littorijia rudis, both very common, both occuring freel}^ at 

 and above high-water mark, and both ranging to a greater or 

 less degree below it. The first of these species is best of all 

 adapted to supply the necessary material, since its range 

 extends from above high-water mark to extreme low-water 

 mark. The second species, though abundant at and above 

 high water, has, at all events on the Dublin Bay shores at 

 Bullock and Sandycove, where the specimens were gathered, 

 but a small downward range. The observations as to exposure 

 in air were accordinglj- chiefly made on Purpura. 



A couple of hours search amongst the high granite rocks 

 along the shore at Bullock yielded some 250 well-grown 

 specimens, all from high-water mark, or well above it; the 

 same number, equally well grown, were taken from low-water 

 mark. All were exposed in air, mouth upward, in a dry room, 

 and after four days, when the animals of all the individuals, 

 both high and low water, had well withdrawn into their shells, 

 they were tested in batches or groups of 10 or 15 or 20 from 



A3 



