416 
MOLLUSCS. 
more often hide in crevices or at the roots of seaweed, mooring themselves by a 
byssus. The shells are very irregular, their form varying according to the hole or 
crevice they inhabit. In the Gastrochamidce, 
forming the last family of this group, Gastrochaena 
comprises bivalves which live buried in the sand. 
These form a long, slender, club-shaped fragile 
tube, covered with adhering particles of sand, and 
divided off by a partition into two portions, the 
anterior containing the shell, the posterior or 
narrower end the siphons. The animal of the 
allied Rocellaria is similar, but forms no tube, and 
has the habit of boring into solid rock, shells, and 
other substances; R. dubia being found in lime- 
animai. (a) and case (b) of Rocellaria. 
stone, and even granite, on the British coasts. 
Suborder Pholadacea. 
The boring Pholadidce and Teredinidce are the only families contained in this 
suborder of the group, the former perforating clay, chalk, limestone, and even 
gneiss. Their shells are always white, thin, but hard and strong, and ornamented 
with prickly rasp-like sculpture. They gape all round the valves, meeting only at 
the hinge and the opposite margin. Accessory plates generally occupy the vacant 
spaces. The valves 
have no hinge-teeth, 
and are not connected 
by a well - defined 
ligament, like most 
bivalves. The 
animals have long 
united siphons, 
fringed at the aper¬ 
tures, and enclosed in 
a tough skin, which 
is often protected by 
cartilaginous cup-like 
processes. In the 
typical Pliolas the 
foot is well devel¬ 
oped, and probably 
forms the principal 
excavating instru¬ 
ment ; the shell being 
used as a file to 
enlarge the crypt as the creature grows. Xylophagci and Martesia bore into 
floating wood. Species of this family are met with everywhere, and about half a 
dozen occur on the British coasts. In some parts of Europe Pliolas is considered 
Pholas in its burrow (nat. size). 
