MKASURKMKNT 



Till'. ri-:RIOSTRACUM 



275 



adductor is partly raised on a similar and vcM-y pi'oiiiiiiont 

 Ibniiation. 



LciKjili and hrrddlli of l)iva,lve sliells is vai'ioiisly measured. 

 Most autliorities measure length , *n' ' autero-posteriur diameter/ 

 by a straight line drawn from the extreme anterior to the 

 extreme posterior margin, and hrcadtli by a, similar line, drawn 

 from the uml)oues 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 mea,sured by the 

 extreme distance of the opposite faces of the closed valves. As, 

 a rule, the length exceeds, and often greatly exceeds, the thick- 

 ness, l)ut in a few cases — e.(j. the Cardissa section of Cardiv/in — 

 this is reversed. 



Tlic. pn'LOstracnm. — Nearly all shells are covcux'd, at some 

 piM'iod of their growth, l)y ix periosfA'acuni} or surface skin, whicli 

 serves the purpose of protecting the shell against the (k'structive 

 effects of the chemical action set up l)y water or air. It also, in 

 some cases (see p. 258), acts as a, kind 

 of base upon wliich tlie shell is ^\e- 

 pijsited. In old shells it is conunonly 

 worn away, especially at those parts 

 which are likely to become abraded. 



The form and composition of thc^ 

 periostracum varies greatly. Some- 

 times (e.g. Oliva) it is a mere trans- 

 parent film, at others {Zonites) it is 

 transparent, l)ut stout and solid. it 

 is corneous in Holenomya, covered 

 with fine hairs in many Helicidae, 

 in Conus, Velutina, and Cantho,rus it 

 i^s thick, fibrous, and persistent; in ^^^ m.- Triton olearium L., 



, TfichotrojnS and Stjme Triton it is Mediterranean, an example of 



p • 1 1 .,, 1 1 ■ i.1 iv shell with a stout ami hairy 



furnished witli long bristles on a periostracum. x .',. 

 thick ground (Fig. 191). In fresli- 



water shells it is usually rather thick, in oidcr 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 tlieir whole length. 



' The term epidermis, as distinct from periostracum, is properly restricted to the 

 outer layer of the skin of the mantle and body generally. 



