CARDIUM. 21 



the potato crop, many of the poorer people subsisted almost 

 entirely on Cockles. 



Cockles found in coarse-bedded sands in the neighbour- 

 hood of tide-ways are often much thicker in the shell than 

 those of confined and sheltered localities. A note, commu- 

 nicated by Mr. R. Ball, on the Arran (Ireland) variety, 

 already mentioned, is illustrative of this. " In 1834," 

 writes that accomplished naturalist, " when visiting the 

 Great South Arran, in company with Mr. Thompson, we 

 found an interesting variety of Cardium edule in a brack- 

 ish lake at the northern end of the island. The shells were 

 remarkably thin and brittle ; the animals were not buried 

 in sand, but appeared to inhabit the Conferva crassa, in 

 which the majority of specimens were found creeping about. 

 I supposed that the thinness of the shell was occasioned 

 by the influence of the fresh water, but finding that the 

 Cockles of the brine-pits are equally thin, this notion must 

 be given up : the probability is, that the cause depends on 

 the quiescent habitat. The greater number of specimens 

 were very small, about a quarter of an inch in diameter 

 a few larger examples were found on the shore of the lake ; 

 they were associated with a very produced variety of Lit- 

 torina jugosaT 



Cardium edule has a wide range, extending southward 

 to the Canary Isles, and a Cockle which cannot be distin- 

 guished from it occurs in the Caspian Sea. In the Arctic 

 seas it is replaced and represented by the Cardium Island- 

 icum. As a fossil it is known in the Red Crag*. 



