"The _Solen_ is furnished with a muscle, called by some writers a tongue. By the aid of this instrument they descend two feet deep in the sand. The tongue is first projected from the shell, and cuts a hole. It then assumes the form of a hook, and draws down the shell into the hole. This operation is repeated until the shell disappears. _Panopæa_ is a large shell--it is in the Museum. * * * * * "4th Family, _Myaria_. _Mya_ _Mya_, or gaper. _Anatina_ _Mya._ "The animal of _Mya_ has also a foot: it buries itself in the sand. You know the broad tooth of the _Mya_ genus. _Anatina_ has a tooth on each valve. * * * * * "Second Section contains four families. "_Mactracea_ has the following genera: _Lutraria_ _Mactra._ _Mactra_ _Mactra._ _Crassatella_ _Mactra_. _Erycina_ ---- _Ungulina_ ---- _Solenomya_ _Mya_. _Amphidesma_ _Tellìna_. "_Crassatella_ is a genus from the seas of New Holland. The shell is very thick, with a brown epidermis. A fossil species is found at Hordwell cliff. There are several species also found in the chalk. _Mactra_, _Lutraria_, and _Erycina_ are found in a fossil state. _Crassatella sulcata_ is common in London clay. * * * * * [Sidenote: CORBULA. LITHOPHAGA.] "The family _Corbula_ contains two genera. _Corbula_ ---- _Pandora_ _Tellìna_. "_Corbula_ comes chiefly from the Asiatic Seas. There is _one_ species, formerly _Mya inequivalvis_, from the British Ocean; fossil species several. _Pandora rostrata_ is British, and is said to be met with at Weymouth. It is a pretty shell. The ligament of these is internal. * * * * * "_Lithophaga_ includes _Saxicava_ _Mytilus_. _Petricola_ _Venerupis_, or Venus of the rocks. [Sidenote: VENERUPIS PERFORANS. PSAMMOBIA.] "These genera consist mostly of small shells, inhabiting stones, into which they bore holes. _S. rugosa_ is British. _Venerupis perforans_ is found on our coasts in stones. The valves of these shells have no accessory pieces like _Pholas_. * * * * * "_Nymphacea_ is the next family, containing, in the first section, _Sanguinolaria_ _Solen_. _Psammobia_ _Tellina_. _Psammotæa_ ---- "In the genus _Psammobia_ we find our _Tellina Feroensis_. (Plate 4.) The shells of this and the preceding genus resemble the solens in a trifling degree, being a little open at the sides. In form they are near _Tellina_, but have not the fold on the anterior valve, but an angle on _each_ valve. The ligament is exterior. * * * * *