Lamellibranchs are classified in various ways by different authorities, the arrangement being based principally on the number and position of the adductor muscles, or on the nature of the gills. For our present purpose we shall look upon them as consisting of two main divisions--the _Asiphonida_ and the _Siphonida_, the former including those species which do not possess true tubular siphons, the inhalent and exhalent openings being formed merely by the touching of the mantle lobes; and the latter those in which the mantle lobes are more or less united and tubular siphons formed. Each of these divisions contains a number of families, most of which have representatives that inhabit the sea; and we shall now note the principal characteristics by which the more important families are distinguished, and take a few examples of each, starting with the _Siphonida_. Examining the rocks that are left exposed at low tide we frequently find them drilled with holes that run vertically from the surface, seldom communicating with each other within, and varying in diameter from less than a quarter of an inch to half an inch or more. Some of these holes are the empty burrows of a boring mollusc, while others still contain the living animal _in situ_. The molluscs in question belong to the family _Pholadidæ_, which contains a number of species that exhibit very remarkable features both as regards structure and habit. The shell is very thin and fragile, but yet composed of hard material, and its surface is relieved by a series of prominent concentric ridges that bear a number of little rasp-like teeth. It gapes at both ends, has neither true hinge nor ligament, and is often strengthened externally by two or more extra or accessory valves. The _hinge-plate_ is a very peculiar structure, for it is reflected over the exterior of the umbones, above which they are supported by about ten thin shelly plates, the whole thus forming a series of chambers. The accessory valves are supported by these bridged structures, and a long, straight, calcareous plate also fills the space along the dorsal side of the shell in some species. The muscular scars and the pallial line are distinctly seen on the inner surface, and a peculiar curved shelly plate projects from under the umbo of each valve. [Illustration: FIG. 132.--_Pholas dactylus_ 1, ventral aspect, with animal; 2, dorsal side of shell showing accessory valves] The animal inhabiting the shell is somewhat wormlike in general form, and the mantle lobes are united in front--that is at the lower end of the shell as it lies in the burrow--except that an opening is left for the protrusion of the short foot. The siphons are united and much elongated, so that they protrude beyond the mouth of the burrow when the animal is active; the gills are narrow, and extend into the exhalent siphon; and the anterior adductor muscle, being very near the umbones, serves the double purpose of adductor and ligament.