BIVALVES, OE COSCIlg.
4. Mya. Gaper. Shell generally gag ing at one end; the hinge with, for the most part, a thick, solid, spreading tooth, not inserted in the opposite valve.
5. Solen. Razor-shell. Shell oblong, gaping at both ends ; hinge with a subu lated reflexed tooth, often double, not in serted in the opposite valve ; the lateral margin rather obsolete.
6. Tellina. The foreside of the shell sloping downwards; the hinge, in gene al, with three teeth; the side teeth, in one of the valves, being either flat or wanting.
7. Cardium. Shell with equal and nearly equilateral valves, longitudinally ribbed, or sulcated, the margin dentat ed ; the hinge having, in the middle, two alternating teeth, one commonly incurv ed, and two lateral, remote teeth, insert ed into each other.
8. Mactra. Shell with equal, but ine quilateral valves ; the hinge a complicat ed, central tooth, with an adjoining pit, and remote lateral teeth, inserted in the opposite valves.
9. Donax. The anterior margin of the shell very obtuse, the margin often cre nulated, two teeth at the hinge, and on the hinder margin a single one, (rarely two, or three, or none,) somewhat re mote.
10. Venus. The lips lying over the anterior margin ; the hinge with three teeth close together, the lateral ones di verging from the apex.
11. Spondylus. With rigid, unequal valves ; the hinge being formed by two recurved teeth, with a cavity between.
12. Chama. The shell heavy; the hinge a gibbous projection, inserted in an ob lique cavity ; the fore parts closed, with out cartilages.
13. Arca. Equivalved ; the hinge with numerous, acute, and alternately inserted teeth.
14. Ostrea. With unequal valves, sometimes eared ; the hinge an ovate ca vity, without teeth, but, in general, with lateral transverse strim ; no space orde pression at the ligament.
15 Anomia. The valves unequal, one flattish, the other gibbous, and one of them often perforated at the base ; the hinge a linear projection, with a lateral tooth, which, in the flat valve, is inserted in the margin ; two bony rays support the animal.
16. Mytilus. Shell coarse, generally affixed by its byssus; hinge generally without teeth, being distinguished by a Lnear, subulated, longitudinal, furrow.
17. Pinna. A sub-bivalve, brittle, erect, and gaping, having a silky beard. The hinge without teeth ; the valves ad here on one side.