138 GASTROCHiENID^. 



Otto Frederic Mliller had long ago correctly delineated its 

 animal. 



The Saxicav^ are borers, although the habit of boring 

 does not seem necessary to their existence, since we find 

 them very commonly free. If there be a crevice, however, 

 in rock, shell, coral, or seaweed, into which they can thrust 

 themselves, they do so ; and if near a limestone rock per- 

 forate it, and form crypts in which they live. Mr. Osier 

 states that, when young they are very active 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. Clark has noticed an instance 

 of their perforating triassic sandstone at Exmouth. Where- 

 ever 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 separations 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 i^robability of 

 such an agent being used by the Pholadida. Mr. Osier, 

 though inclined to such a view, could detect no acid, nor, 

 for reasons previously stated, is it likely. Mr. Hancock, as 



