120 
NATURAL HISTORY. [NEW BUILDING. 
a very compressed body and thin nearly transparent 
shells ; the cartilages are placed on the edge of two di¬ 
verging ridges on one of the valves, which fit into two 
grooves in the other. These shells are sometimes used 
as glass to glaze windows. 
The family of Anomiadce have the thin pellucid shell 
of the former, but the body is usually rather more convex, 
and they are attached to marine bodies by a peculiar mus¬ 
cle, which passes out through a notch in front of one of 
the valves; this muscle after a time secretes on the sur¬ 
face to which it is affixed a stony substance, formed of 
longitudinal shelly plates, probably deposited between the 
fibres of the muscles, which has been called a stopper, and 
by some considered as a third valve. In Anomia this 
stopper is free. In Placunonomia it is fixed in the notch, 
which is obliquely prolonged as the shell enlarges. These 
shells become gradually moulded to the surface they rest 
on. Thus, if the shell is found on a Pecten, it is ribbed, 
and if on the spine of an Echinus, or the stem of a sea¬ 
weed, it is compressed and subcylindrical. 
The three remaining Classes of Mollusca have no foot 
or only a rudimentary one 
The third class of Brachiopodous Mollusca (Braciiio- 
poda, Case 38) are inclosed by two regular shelly valves, 
one placed on the back and the other on the lower surface 
of the body, which are quite free from each other, or only 
united by interlocking teeth on the hinge margin*. They 
have no distinct head, but the mouth is placed on the 
hinder part of the cavity between the leaves of the mantle, 
and is furnished with two long spirally twisted arms, by 
which they reach their food; the organs of respiration 
are placed on the edge of the mantle. All these shells 
are attached to marine bodies. 
The family of Terebratulidte are regular, and somewhat 
like a Grecian lamp in form, and have therefore been 
called Lamp-shells. The valves are articulated together, 
and are attached by means of a tendinous band, which 
passes out of the hole in the apex of the upper valve, 
as in the Terehraiulce and Spirifer . 
The LinguVidce are attached by a tendinous tube, re¬ 
sembling the stem of the Barnacles, which projects between 
the apex of the gaping valves. 
