102 Mr Stark on Ticu Species of' Pholas 



pear-shaped cavity at the bottom, the largest diameter of the 

 shell being undermost. Where the Pholades are crowded to- 

 gether, which is generally the case, the divisions between the 

 different cells are often extremely thin, and in some this par- 

 tition is completely removed. The direction of the bore is 

 not always vertical, though nearly so ; but in some instances, 

 where the rock had been broken down to an angle, or round- 

 ed, the Pholades were found at various inclinations, corre- 

 sponding to the surfaces of the stone. 



From repeated examination of the recent animals and then- 

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

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

 rasping of the stone with their valves. Indeed, I am sur- 

 prised how any one who has seen these animals in their na- 

 tive rocks could for a moment think otherwise ; for in the 

 Joppa specimens, circular lines are distinctly visible in the 

 cell of the animal, corresponding to the elevated striae on the 

 shell, and presenting the appearance as of having been bored 

 by an auger. Pennant remarks the same appearance in the 

 cells of the Pholades found by him on the English coast, as 

 Bonanni 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. 



It has been held as a presumption against the Pholades 

 perforating rocks by a mechanical operation, that some of the 

 species have shells nearly smooth, and unfitted for such a pur- 

 pose ; and the Mya Pholadia and Mytilus lithophugus are 

 produced as instances where it is next to impossible that, 

 without the aid of a solvent fluid, such animals could form 

 protecting cells in hard substances. From not having seen 

 the animals alluded to alive, and in their native habitations, 

 it would be presumption in me to give a decided opinion on 

 the subject. But, reasoning from analogy in the structure of 

 the animals, and the habits of such as have been observed, it 

 infers no impossibility to conceive that they penetrate rocks in 

 In similar manner. Little asperity in the instrument is re. 



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