THE SAXICAVA. 103 



species, the Fork-tailed Date Shell (Lithodomus eaudigera), is able 

 to bore into substances which the pholas cannot penetrate. It is 

 truly a wonderful little shell. Some of the hardest stones and 

 stoutest shells are found pierced by hundreds of these curious 

 beings, which seem to have one prevailing instinct, namely, to 

 bore their way through everything. Onwards, ever onwards, 

 seems to be the law of their existence, and most thoroughly do 

 they carry it out. They care little for obstacles, and if one of 

 their own kind happens to cross their path, they quietly proceed 

 with their work, and drive their tunnel completely through the 

 body of their companion. 



The precise method employed in excavation is at present un- 

 known, for the shape of the shell, and the exactitude with which it 

 fits the burrow, prove that the mollusc does not form its tunnel 

 by means of the protuberances on the surface of the shell, and 

 no other method of boring has at present been discovered. 



There is another notable burrower among the bivalve marine 

 shells, which is remarkable for the depth to which it bores, and 

 the hard nature of the substances through which it makes its 

 way. This is the shell called Saxicava rugosa ; one of the most 

 variable of the molluscs, so variable indeed, that no less than 

 fifteen different names have been given to it, each being supposed 

 to be a separate species. Not only species, but even genera have 

 been formed from the varieties of this curious shell. It is a flattish 

 bivalve, of no very great size, symmetrical in shape when young, 

 but oblong when old. 



This creature burrows as rapidly as the species which has 

 just been described, and the process by which the feat is accom- 

 plished is quite as enigmatical. Several conchologists have ex- 

 pressed an opinion that the animal must secrete some liquid 

 solvent, which softens the rock, and permits the shell to pass. 

 But, although it is possible for the boring snail to excavate by such 

 means, the Saxicava can hardly do so. For the boring snail is a 

 terrestrial species, and would be able to employ a solvent with- 

 out interruption ; but as the Saxicava works below the surface 

 of the sea, any solvent which it could employ would be either 

 washed away, or so diluted by the water that it would have no 

 effect upon the stone. 



Still, that the creature must employ some means not yet 



