43 



MOLLUSCOIDEA. 



A 



TJiecidium mediterraneum. A, Nat. size 

 Section through the shell (magnified). 



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 



