CHAPTER X. FOSSIL SHELL-FISH (MOLLUSCA). =Molluscan Characters.--= The phylum or sub-kingdom Mollusca is a group of soft-bodied animals (mollis, soft), which, although having no external skeleton, usually possess the protective covering of a shell. This shell is secreted from the outer skin or mantle, and is composed of carbonate of lime (calcareous) with a varying proportion of organic material. =Hard Parts.--= Fossil molluscan remains consist practically of the shells, but the calcareous apertural lid (operculum) of some kinds is often preserved, as in _Turbo_ and _Hyolithes_; or the horny lids of others, as _Bithynia_ of the European Pleistocene "brick earths." The cuttle-fishes have hard, horny beaks and internal bones, and the latter are frequently found fossil in Australia. =Characters of Pelecypoda.--= The class for first consideration is the important one of the Bivalved Mollusca, the _LAMELLIBRANCHIATA_ ("plate-gills") or _PELECYPODA_ ("hatchet foot"). The shells are double, hinged dorsally and placed on either side of the animal, that is, they are left and right. The height is measured on a vertical line drawn from the beaks or umbones to the ventral margin. The length is the greatest distance between the margins parallel with a line drawn through the mouth and posterior adductor impression. The thickness is measured by a line at right angles to the line of height. The shell being placed mouth forward, the valves are thus left and right. The anterior is usually shorter, excepting in some cases, as in _Donax_ and _Nucula_. =Hinge Structure.--= In the absence of the animal, the character of the hinge-structure is very important. Some are without teeth (edentulous). The oldest forms have been grouped as the "Palaeoconcha," and it has been shown that here, although well-developed teeth were absent, the radial ribs of the surface and ventral areas were carried over to the dorsal margin and became a fixed character in the form of crenulations or primitive teeth. The taxodont type of hinge teeth shows alternating teeth and sockets, as in _Nucula_. The schizodont type is seen in the heavy, variable teeth of _Trigonia_ and _Schizodus_. The isodont type of hingement is a modification of the taxodont, represented by two ridges originally divergent below the beak, and forming an interlocking series of two pairs of teeth and sockets as in _Spondylus_; or where the primitive hinge disappears as in _Pecten_, the divergent ridge-teeth (crura) may only partially develop. The dysodonts have a feeble hinge-structure derived from the external sculpture impinging on the hinge-line, as in _Crenella_. The pantodonta are an ancient palaeozoic group which seems allied to the modern teleodont or long toothed shells, but the laterals may exceed a pair in a single group, as in _Allodesma_.