434 Mr STARK on Two Species of Pholas 



widen as they recede downwards, corresponding to the animal's 

 growth. The Pholas itself is found in an inverted pear-shaped 

 cavity at the bottom, the largest diameter of the shell being un- 

 dermost. Where the Pholades are crowded together, which is 

 generally the case, the divisions between the different cells are 

 often extremely thin, and in some this partition is completely 

 removed. The direction of the bore is not always vertical, though 

 nearly so ; but in some instances, where the rock had been bro- 

 ken down to an angle, or rounded, the Pholades were found at 

 various inclinations, corresponding to the surfaces of the stone. 



From repeated examination of the recent animals, and their 

 perforations, I have no hesitation in asserting, that these two 

 species, at least, form their holes by the rotatory motion or rasp- 

 ing of the stone with their valves. Indeed, I am surprised how 

 any one who has seen these animals in their native rocks could 

 for a moment think otherwise ; for in the Joppa specimens, cir- 

 cular lines are distinctly visible in the cell of the animal, corres- 

 ponding to the elevated striae on the shell, and presenting the 

 appearance as of having been bored by an auger. PENNANT re- 

 marks the same circumstance in the cells of the Pholades found 

 by him on the English coast, as BON ANN i had formerly done in 

 the Italian specimens. These marks, indeed, disappear in the 

 upper part of the perforation, from the friction occasioned by 

 the expansion and contraction of the rugous tube ; but in the 

 cavity where the Pholas lodges it is always distinctly, and often, 

 especially when the animal is large, prominently marked. 



Specimens of the shells, from the locality mentioned, are now 

 submitted to the Society, along with portions of the shale in 

 which they are found. 



It has been held, as a presumption against the Pholades per- 

 forating rocks by a mechanical operation, that some of the spe- 

 cies have shells nearly smooth, and unfitted for such a purpose ; 

 and the Mya Pholadia and Mytilus lithophagus are produced as 



