THE SHELL-FISH OF THE COAST. 89 
ing the shell of Petricola pholadiformis, and in which, 
after the completion of the work of boring, the 
projecting points on the shell are perfectly clear and 
sharp, and not rubbed down as we should naturally 
expect to find them after the hard work of rock- 
abrasion. Such ashell is the Pholas, of which three 
species are represented on the New Jersey coast. 
The largest of these, Pholas costata (Pl. 2, Figs. 21- 
23), which measures about six inches in length, is 
very generally represented only by fragments of the 
shell, and it is still doubtful whether it normally 
inhabits this part of the coast. It is more properly 
a species from the South (although observed as 
far north as New Bedford), where it may be found 
at some little distance beneath the surface in the 
mud-banks which are exposed at low water. The 
other two species, Pholas truncata and P. crispata 
(Pl. 8, Figs. 1, 2), are very much smaller, and, while 
their shells are ribbed anteriorly, they lack the full 
series of spinose riblets which so regularly diversify 
the shell of the larger form. In the latter species 
the shell ‘ gapes’ broadly at the two ends, the pos- 
terior opening permitting of the extension of a very 
powerful and muscular siphon. It is a common 
habit with Pholas crispata to bore into rock, and 
specimens of bored-rock fragments in museums 
are not rare. The collections of the Academy 
of Natural Sciences of Philadelphia contain a re- 
markable block of gneiss which is bored in this 
manner. It has been suggested that the process 
of boring may be entirely performed by the foot 
taking up particles of sand and rubbing these 
