724 CLASS XII. 
of mantle, mostly lamellose, two on each side, more rarely pectinate, 
composed of thin fimbriz or laciniz crowded together. Four lamelli- 
form tentacles in by far the most, oval or triangular, placed in pairs 
at the sides of mouth. ‘'T'wo lateral shells incumbent on mantle, 
conjoined by an elastic ligament at the back. 
On the internal structure of these lamellibranchiate conchifers, 
what was needful has been already said above. The beard-threads, 
named byssus, still require a short notice. At the foot of the 
conchifers (see above, p. 717) there is often situated a bundle of hairs 
or filaments to which the name of byssus has been given ; sometimes 
these filaments are united into a common mass; they are attached 
in a cavity at the base of the foot by a peculiar substance secreted 
there (conjunctive matter). From the cavity a furrow runs to 
nearly the extremity of the foot, and along this furrow is situated 
the glandular tissue by which the byssus is secreted. REAUMUR 
maintained that these horny threads were spun just like the web of 
caterpillars and the nets of spiders. With the assistance of the 
muscular foot these threads are directed to determinate situations 
and caused to adhere there firmly by their broad disciform extremi- 
ties ; they grow again if cut off. 
Comp. A. MUELLER de Bysso Acephalorum Acced. Tab. 11. Berolini, 
1836, 4to, and in WinGMANN’S Archiv, 1837, 8. .—39, Tab. I. 11. Various 
opinions of writers on these filaments were noticed in the first edition of 
this Handbook, 11. blz. 37. 
Several conchifers, which are not attached by byssus-filaments, 
grow fast by one of the two valves to rocks or different substances 
found in the sea. That some of them move freely and get over the 
ground with their foot has been already noticed (p. 717). The two 
valves of the shell are separated from each other by the elastic 
ligament situated at the hinge behind the point (p. 682). White 
compact muscular fibres, which run transversely from one valve to 
the other, close the shell on the other hand. These bundles leave 
impressions on the interior of the shell, two on each valve, the one 
before, the other behind, if, as in most of these animals, two adductor 
muscles be present; if, on the contrary, as in the oyster, there be 
only one, then only a single muscular impression is to be seen on 
each valve, situated nearly in the middle. 
Pott made many experiments with regard to the force of these muscles 
in living conchifers. Thus he states, for instance, that the adductor muscles 
in the animal of the Noah’s shell (Pectwnculus) can overcome the resistance 
of solbs., and that more than 70 lbs. are required to tear them asunder. 
The shell weighed only one pound, the animal itself only 4 ounces. 
