Page:Natural History, Mollusca.djvu/309

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animals, and soon commence to perforate. Both that gentleman and Mr. Garner have noticed that their excavations are not round, nor the sides smoothed off, like those of the holes made by Pholas. As for us, we only know of their boring into calcareous rocks, but Mr. Clarke has noticed an instance of their perforating triassic sandstone at Exmouth. Wherever we have a sea-coast of mountain limestone, the surface of the rocks is almost invariably found riddled by Saxicava. The whole front of the Plymouth breakwater has been attacked by it, and much alarm for its safety excited. Mr. Couch observes that the Saxicava never bores deeper than six inches, and that, consequently, unless a new surface be exposed by the destruction of the perforated part, there is not much danger. Owing, however, to the thinness of the partitions, which often are the only separation between the crypts of these mollusks, there is a great probability of the action of the sea rapidly forming new surfaces in such cases. How they bore has been as much discussed as the question how Pholas bores. The general opinion has been, that Saxicava bores by means of an acid secretion; an opinion held by many who will not admit the probability of such an agent being used by the Pholadidæ. Mr. Osier, though inclined to such a view, could detect no acid, nor, for reasons previously stated, is it likely. Mr. Hancock, as we have seen when treating of Pholas, expressly asserts that the Saxicavæ. bore by rasping, effected by means of siliceous particles contained in the anterior part of the mantle. Mr. Couch entertains a similar view. We have not been able to satisfy ourselves of the presence of