IX MEASUREMENT — THE PERIOSTRACUM 275 



adductor is partly raised on a similar and very prominent 

 formation. 



Length and breadth of bivalve shells is variously measured. 

 Most authorities measure lengthy 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} 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 de- 

 posited. 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. Some- 

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

 parent film, at others QZonites) it is 

 transparent, but stout and solid. It 

 is corneous in Solenomya^ covered 

 with fine hairs in many ITelicidae, in 

 Conus, Velutina, and Oantharus it is 

 thick, fibrous, and persistent; in Tri- ^^^^ iqi. -rriton oiearium l., 



chotropis and some Triton it is fur- Mediterranean, an example of 

 .-IT •-! 1 1 • ii j^i • 1 a shell with a stout and hairy 



nished with long bristles on a thick periostracum. x ^. 

 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. 



1 The term epidermis, as distinct from periostracum^ is properly restricted to 

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



