Passing now to the _Asiphonida_, we deal first with the family _Arcadæ_. These include a number of shells which, though very variable in general form and appearance, may all be recognised by the long row of similar comb-like teeth that form the hinge. The shells of this group are regular in form, with equal valves, and are covered with epidermis. The mantle of the animal is open, the gills are united by a membrane behind, and the foot is large, curved, and grooved. [Illustration: FIG. 148.--_Pectunculus glycimeris_, WITH PORTION OF VALVE SHOWING TEETH, AND _Arca tetragona_] One of the prettiest shells in the family is _Pectunculus glycimeris_, which reaches a length of about two inches. The shell is grooved in the direction of the lines of growth, and there are also very delicate striations running radially from umbones to margin; and the ground colour of white or pale yellowish is beautifully mottled with reddish brown. We give a figure of this species, together with a drawing of the peculiar and characteristic teeth, but a more typical shell of this family may be seen in the Noah’s Ark (_Arca tetragona_). This shell is almost quadrate in form, swollen, and strongly ribbed. The hinge is straight, with many comb-like teeth--increasing in number with the age of the shell; and the umbones are separated by a diamond-shaped ligament. The foot of the animal is heeled--that is, it has a creeping surface that extends backwards as well as forwards; the mantle is furnished with minute eyes (_ocelli_), and the animal has two distinct hearts. We give a figure of this peculiar shell, and the other British members of the same genus, though varying more or less in form, may be recognised at once by the same general characteristics. In the same family we have the small nutshells (genus _Nucula_), which are often dredged up from deep water in large numbers; and the elongated shells of the genus _Leda_, also inhabitants of deep water; and, as before stated, the affinities of all may be readily established by the characteristic nature of the teeth. We now pass on to the family of Mussels (_Mytilidæ_), of which the common Edible Mussel (_Mytilus edulis_) is a typical species. In this interesting group the shell is oval or elongated, with equal valves, and is covered with a dark-coloured epidermis which is often distinctly fibrous in structure. The umbones are at the anterior end of the shell, which end is usually very narrow and pointed, while the posterior is broad and rounded. The hinge has small teeth or none, and the ligament, which is long, is internal. The shells of mussels consist of two distinct layers; on the inner, which is often of a most beautiful pearly lustre, may be traced the simple pallial line and the impressions of the small anterior and the large posterior muscles.