430 
MOLL USCOLDEA. 
calcareous framework nowhere rises freely from the lid, with which it is connected 
by a calcareous network. In the section (B) given in the illustration we see in the 
dorsal valve the depression for the hinge on which the valve rotates. The shell is 
opened by the muscles (b) which run from the bottom of the ventral valve to a 
process directed backward in the dorsal valve behind the hinge. It is closed by 
the muscles marked a, which lie in front of the hinge. The shells of Thecidium 
become attached to submarine objects, and 
are brought up in considerable numbers by 
the nets of the coral - fishers between the 
Gulf of Bona and Cape Rosa, from a depth 
of from forty to fifty fathoms. The number 
of specimens of Terebratula is small as 
compared with that of Thecidium, twenty 
to thirty specimens of the latter being 
often found together. When first caught, 
Thecidium opens its valves very wide, but 
when isolated and placed in small vessels 
gapes less widely. The small dorsal valve 
or lid can be raised to form a right angle 
with the other valve, but, when the slightest 
movement is made, it snaps to with the 
speed of lightning. These lamp-shells are 
undoubtedly sensitive to light, even a shadow thrown upon them making them 
close their shells instantly. On account of the wide gape, the inner organs, 
such as the cirri and arms, can be accurately observed. The inner surface of the 
shell on which the mantle lies is so dazzlingly white, and the latter so transparent, 
that the calcareous framework and the prominences on the valve are as easily seen 
as if there were no intervening mantle. Externally, the shell is rarely white and 
smooth, being usually covered with plants or animals which have attached 
themselves to it, and the valves perforated in all directions, chiefly by boring 
sponges. 
The Rhynchonellidce, or beaked Brachiopods, were extremely numerous in 
the oldest geological times, but are now only represented by some three genera. 
The most important genus is Rhynchonella , which is one of the oldest and most 
widely distributed of all known organisms, being found from Silurian times through 
all subsequent strata. The living R. psittacea shows best the characteristic beak¬ 
like process of the ventral valve ; the aperture for the stalk being found under this 
beak. The valves are fastened together as in the Terebratulidce, but the calcareous 
framework consists merely of two short, narrow plates, which are attached to 
the smaller valve. Rhynchonella is not very numerous in northerly regions, 
but empty valves are found in mud. Observations on the living animal are 
rendered difficult by the fact that it is peculiarly sensitive to all disturbance, 
and closes its valves at the slightest movement. The arm - spirals widen 
sufficiently to allow the cirri to reach the edge of the shell; the arms do not 
appear capable of unrolling and protruding beyond the shell. The members of 
another family of this order of hinged Brachiopods, the Spiriferidce , are rendered 
Thecidium mediterraneum. A, Nat. size; B, 
Section through the shell (magnified). 
