_Length and breadth_ of bivalve shells is variously measured. Most authorities measure _length_, or ‘antero-posterior diameter,’ by a straight line drawn from the extreme anterior to the extreme posterior margin, and _breadth_ by a similar line, drawn from the umbones to a point, not very clearly marked, on the opposite ventral margin (see Figs. 184 and 185). Others, less correctly, reverse these terms. _Thickness_ is measured by the extreme distance of the opposite faces of the closed valves. As a rule, the length exceeds, and often greatly exceeds, the thickness, but in a few cases--_e.g._ the _Cardissa_ section of _Cardium_--this is reversed. _The periostracum._--Nearly all shells are covered, at some period of their growth, by a _periostracum_,[351] or surface skin, which serves the purpose of protecting the shell against the destructive effects of the chemical action set up by water or air. It also, in some cases (see p. 258), acts as a kind of base upon which the shell is deposited. In old shells it is commonly worn away, especially at those parts which are likely to become abraded. The form and composition of the periostracum varies greatly. Sometimes (_e.g._ _Oliva_) it is a mere transparent film, at others (_Zonites_) it is transparent, but stout and solid. It is corneous in _Solenomya_, covered with fine hairs in many _Helicidae_, in _Conus_, _Velutina_, and _Cantharus_ it is thick, fibrous, and persistent; in _Trichotropis_ and some _Triton_ it is furnished with long bristles on a thick ground (Fig. 191). In fresh-water shells it is usually rather thick, in order to protect the shell from the erosive powers of certain kinds of water. In some cases (_Mya_, _Anatina_) the periostracum is continued over the siphons, so as to form a protection throughout their whole length. [Illustration: FIG. 191.--_Triton olearium_ L., Mediterranean, an example of a shell with a stout and hairy periostracum. × ½.]