54 BIVALVE SHELLS. 



exceeding the length of their, valves ; these never voluntarily 

 quit their abodes, and often become buried and fossilized in thena. 

 They most usually burrow in soft ground, but also in coarse 

 gravel, and firm sands and clays ; one small Modiola makes its 

 holes in the cellulose tunic of Ascidians, and another in floating 

 blubber. 



The boring shell-fish have been distinguished from the mere 

 burrowers, perhaps without sufficient reason, for they are found 

 in substances of every degree of hardness, from soft mud to 

 compact limestone, and the method employed is probably the 

 same. 



The means by which bivalves perforate stone and timber has 

 been the subject of much inquiry, both on account of its physio- 

 logical interest, and the desire to obtain some remedy for the 

 injuries done to ships, and piers, and breakwaters. The ship- 

 worm (Teredo) and some allied genera, perforate timber only; 

 whilst the Pholas bores into a varietj^ of materials, such as chalk, 

 shale, clay, soft sandstone and sandy marl, and decomposing 

 gneiss ; it has also been found boring in the peat of submarine 

 forests, in wax and in amber. It is obvious that these substances 

 can only be perforated alike by mechanical means ; either by the 

 foot or by the valves, or both together, as in the burrowing 

 shell-fish. The Pholas shell is rough, like a file, and sufficiently 

 hard to abrade limestone ; and the animal is able to turn from 

 side to side, or even quite around in its cell, the interior of which 

 is often annulated with furrows made by the spines on the front 

 of the valves. The foot of the Pholas is very large, filling the 

 great anterior opening of the valves ; that of the ship-worm is 

 smaller, but surrounded with a thick collar, formed b}^ the edges 

 of the mantle, and both are armed with a strong epithelium. 

 The foot appears to be a more efficient instrument than the shell 

 in one respect, inasmuch as its surface may be renewed as fast 

 as it is worn away. 



In the museum of the Academy of Natural Sciences of Phila- 

 delphia are specimens of Pholas dactylics from the coast of France 

 imbedded in gneiss rock, much too hard to have been excavated 

 by the valves of the shell ; moreover the rough points of its 

 sculpture show no signs of abrasion, which must surely have 

 taken place if these delicate projections had been used to rasp 

 the surrounding surface. 



The mechanical explanation becomes difficult in the case of 

 another set of shells, Lithodomus, Gastrochsena, Saxicava, and 

 Ungulina, which bore into calcareous rocks, and attack the 

 hardest marble, and still harder shells. In these instances the 

 valves can render no assistance, as they are smooth, and covered 

 with epidermis ; neither does the foot help, being small and 

 finger-like, and not applied to the end of the burrow. Their 



