GALLERY.] NATURAL HISTORY* 139 
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 it in its place ; 
and when it requires to enlarge its tubular dwelling, 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 Fistulana and Gastrochcena 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 distorted. 
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 prominent fulcrum. In Solen and Ensis, the foot is 
club-shaped, and the tubes 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 w T hen irritated. In Cidtellus 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 
produced, very thick and black, and the tubes are united. 
Ctenoconcha , which has many characters in common with 
the Solens, has the teeth like Nucula , but it has an ex¬ 
ternal cartilage. 
The family of Anatmidce have a broad compressed foot; 
the edges of the mantle-lobes are thickened, and the sy¬ 
phons are elongate and more or less united ; the shells 
are oblong, thick, and brittle, covered with a granular or 
