PHOLAS. 



103 



putrid state. He attributed the phenomenon (which he 

 considered a ' ' vrai phosphorus naturel ") to a fermen- 

 tation, resulting from the breeding-season ; and he 

 supposed that it was analogous to the cases of the male 

 glowworm and centipede. These experiments were 

 made in autumn, and at other times of the year when 

 the weather was not very warm. Dr. J. M. Davis 

 examined P. dactylus at Tenby in the autumn of 1840 ; 

 but although he kept it alive and in a vigorous state for 

 many weeks, it never was luminous or phosphorescent. 

 Out of fifteen living individuals of this species obtained 

 by M. Cailliaud at the end of April and in December 

 1854, ten or tw r elve shone in the dark. In none of these 

 did the foot exhibit any light; only the mantle and 

 siphons, which when rubbed with the finger were ex- 

 tremely phosphorescent, and shone even through the 

 shells. The siphons were furnished with it in such 

 quantity, that he w r as able to trace with them bright 

 marks on a table. He endeavoured, but in vain, to find 

 the same property in other perforating mollusks. I 

 am disposed to believe that this luminosity is caused 

 not by the Pholas, but by extraneous microscopic 

 organisms. The subject ought to be further investi- 

 gated. M. Necker has show r n that the shell of Pholas, 

 as well as of several other mollusca, is formed of arra- 

 gonite ; and inasmuch as that mineral slightly exceeds 

 calc spar in specific gravity (the proportion being 2 - 9 to 

 2'7 or 2*8), he came to the conclusion that Pholas 

 excavates calcareous rocks by means of the prickles 

 with which the shell is furnished, aided by an acid. 

 But he placed Helix nemoralis and Mytilus edulis in the 

 same mineralogical category with Pholas, and ascribed 

 a still greater density to the common oyster. It is also 

 important to notice that the impurity of most calcareous 



