ARCA.
PECTUNCULUS.
NUCULA.
NAYADA.
"The hinge of Arca in this arrangement is always straight, furnished with a number of teeth; the ligament is external. The shells are open at one end, for the animal throws out at the aperture a number of threads, by which it fastens itself to the rocks. The species are thirty-seven, and also several fossil.
"The orbicular form of Pectunculus, and its arched hinge, distinguish this genus from the preceding one. They are allied to the Pectens by their form, and their crenulated internal margin.
"The hinge of Nucula is set with little teeth on each side, like a comb. It is pearly within, and sometimes small pearls are found in the shell. Pectunculus costatus is found in London clay.
"Trigoniana is a small family containing Trigonia and Castalia. The first is a fossil genus chiefly. Some species are found in the Portland stone, or oölite beds.
"The next family contains the Nayada, chiefly composed of fluviatic, or fresh-water shells. They are covered with an olive-brown epidermis, which is constantly found eroded, or destroyed at the beaks. The muscular impressions are lateral and much separated; one of them is formed of two or three distinct irregular impressions.
| Unio, taken chiefly from | Mya. |
| Hyria | Mya. |
| Anodon | Mytilus. |
| Iridina | Very rare genus. |
UNIO.
ANODON.
DICERAS.
"Unio has two teeth on each valve; one is cardinal, the other lengthened out. The ligament is exterior—the shell pearly. Unio pictorum is common in rivers. The shell is used to hold small masses of gold or silver for artists, under the name of shell-gold.
"Anodon is also to be met with in our rivers.