"The _Pholas_ has the power of emitting a phosphoric liquor, which shines with brilliancy in the dark. "I must remind you that the accessory valves are fixed to the margin of the shell by a gelatinous substance; this decays after the death of the animal, and consequently the smaller valves are frequently wanting. "The number of species is twelve. Several of them are found on our coasts. The _Pholas_ genus is without colour, but the reticulations in some species are exceedingly delicate. _Pholas candida_ (Plate 2) is found on the shores of Kent; you will be pleased with the shells. _Pholas dactylus_ is larger and coarser, and not at all uncommon. "We have now finished our first division, and must proceed to the bivalves." CHAPTER IV. Second Division.--_Bivalves_: 14 genera. "I fear," said Charles, "that this new division will be rather difficult, for my father tells me that we must pay particular attention to the _hinges_ of bivalve shells." "Then _apply_ yourself to the study of hinges, Charles, and your difficulties will chiefly disappear," answered Mr. Elliot. [Sidenote: MYA.] "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. [Sidenote: 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.)