112 
NATURAL HISTORY. [NEW BUILDING. 
to bore deeper and deeper into the wood, and lines the 
holes as he proceeds with a shelly tube for his protection. 
The family of Gastrochcenadce , have animals very similar 
to the former; the valves are thin, gaping in front, and 
united by a narrow ligament with a thin marginal carti¬ 
lage. They live inclosed or sometimes imbedded in the 
substance of a shelly tube, which is formed by the 
animal to protect its elongated and partly naked body from 
the roughness of the sand, or the rock in the holes of 
which they reside. The animal is provided with a series 
of filaments in front, which it emits to ascertain what is in 
its neighbourhood, and for the protection of which it forms 
tubes that serve as anchors to keep the tube of the body in 
its place; and when it requires to enlarge its tubular dwell¬ 
ing, it adds new matter to the outer or hinder extremity of 
the tube, having previously absorbed any of the old tube 
that may be in its way. When this is not the case, the 
expanded mouth of the old tube forms ruffles on the tube 
beneath the last formed aperture. In Aspergillum both 
the valves are imbedded in the lower part of the tube, 
so that only their umbones can be seen on the outer side. 
In Clavagella one valve is imbedded and the other is 
free, while in Fistuland and Gctstrochcena both the valves are 
regular and free; in the former the tube is regular and 
club-shaped, and in the latter it is irregular and dis¬ 
torted. 
The family of Solenidce have a very elongate club-shaped 
foot, and elongate subcylindrical valves gaping at each end ; 
the hinge is formed by two or three compressed teeth in 
each valve, the hinder of which is bifid, and the cartilages 
are external, linear, and supported on a large very pro¬ 
minent fulcrum. In Solen and Ensis , the foot is club- 
shaped, and the syphons are short and united. In Pharus , 
the foot is long with a dilated end, and the syphons are 
elongate and separate. In Solecurtus the foot is ovate, 
compressed, very large, and the syphons very long, united 
together beneath, and distinct at the top, where they often 
separate into rings when irritated. In Cultellus the foot 
is thick and pointed, and the two syphons are separate 
nearly to the base. In several the periostraca is hard and 
produced beyond the margin ; in Glycimeris it is much 
