388 



living specimens, are comparatively scarce in these same localities 

 and such as are found are generally in a bad state of pre- 

 servation. 



The specimens of Saxicava arctica from the East- Greenland 

 locality Heklahavn [Hekla-Harbour] have, as a rule, lost the 

 periostrakum almost completely, and Mr. Jensen found that the 

 shells are either extremely thick, so as to completely disfigure 

 the animal, or else they are thin and extremely fragile. He is 

 of opinion thai this indicates that a struggle is going on between 

 the animals and some lime-dissolving agent, and that some of 

 them are able to hold their own in this struggle by a greatly 

 enhanced production of shell-substance, while others succumb. 



Carbonic acid is obviously the only dissolving agent that 

 can be present in normal seawater, since all stronger acids, if 

 by any chance they were formed, must immediately combine 

 with the alkali and liberate carbonic acid. If therefore a high 

 tension of CO^ should be demonstrated in those localities, 

 where the bivalves possess the characters observed by Mr. Jensen, 

 his views would be thereby corroborated. 



The only locality which I was able to examine at the time 

 was the Baltic near the Danish Islands of Bornholm and 

 Christiansø. 



The table on page 389 shows that from a depth of about 

 50 m. downwards the tension of carbonic acid rises rapidly and 

 attains a considerable height at the bottom. It is in perfect 

 agreement with this fact that dead shells are readily dissolved 

 and that some, at least, of the living Molluscs protect them- 

 selves by means of a thick periostrakum. 



The correctness of Mr. Jensen's opinion cannot of course 

 be finally established by observations in one locality but it 

 seems to me that he is most probably right and that interesting 

 results are likely to be obtained by combined examinations in 

 many localities of the tensions of carbonic acid and the structure 



