"The hinge of Mya, the first on the list, is easily known. The generic characters are, shell gaping at one end, hinge mostly with one thick spreading tooth, not inserted into the opposite valve. The Mya race burrow in the sand. Here is Mya arenaria, a large thick shell, frequent on the shores of Kent: the large tooth is sufficiently plain in this species. Mya truncata ([Plate 4]) is as common, and the curious membranous case, which you will find attached to one end of the shell, is a guide to the species. Both these species are without colour, and have little to attract in their outward appearance. The genus, however, according to Linnæus, varies exceedingly, and contains forty-one species.
SOLEN.
TELLINA.
"Solen. Shell bivalve, open at both ends, tooth of the hinge subulate, or awl-shaped, reflex, often double.
"In this genus, the great length, in comparison with the breadth of the shells in many of the species, is remarkable: some are exceedingly brittle. Our example, Solen siliqua ([Plate 1]) is a British species. The hinge is not in the centre of the shell, but nearer to one end of it. Some are shaped like the handle of a knife or a razor, others are bent resembling the blade of a scimitar. The Solen lives in the sands of the sea-shore, often burying itself two feet deep, and retaining its shell in a vertical position: thirty-five species.
"The genus Tellìna is remarkable for the beauty of the shells, and, according to the arrangement of Linnæus, contains ninety-seven species. The exterior is sometimes marked with radiations: the surface of some shells is very finely polished, while in others it is covered with striæ and undulations. The species that you have placed before me, Lucy, is Tellìna Feroensis; the shell is finely striated, and has also radiations. ([Plate 4.])
"The generic characters are chiefly these: shell compressed towards the anterior slope, teeth of the hinge mostly three, the lateral ones smooth, in one valve. Two or three small species are common on our coasts. I should also observe that there is a convex fold on one valve and a concave fold upon the other. Many of the Tellìna genus are found buried in the sea-sands.
CARDIUM.
MACTRA.
"Cardium. Generic character: shell equivalve, convex, ribbed, striated, or grooved, the margin toothed: hinge with two teeth near the beak, and a lateral one on each side: fifty-four species.
"Observe how the beaks of this common Cardium, cockle, turn inwards, and the bosses project. Another striking character is the ribs, that are generally longitudinal, and not concentric or transverse, as in Tellìna, and, as you will see, in Venus. C. aculeatum has small spines on the valves; C. costatum, the ribbed cockle, is one of the finest species of this genus, and C. cardissa is a beautiful shell. ([Plate 4.]) The common cockle is Cardium edule.
Mactra. Generic character: shell bivalve, unequal sided, middle tooth of the hinge complicated, with a small hollow on each side, and lateral side-teeth: thirty-seven species.