62 Shell Life 



streaks or spots of red-brown, and by a number of 

 fine overlapping scales. It is a common but little- 

 known species, its range being only between the 

 depths of 10 and 86 fathoms. Both these species 

 appear to owe their security alike from storm and 

 enemy to the thin flat form that sits so tightly 

 on its chosen base, and affords little opening to the 

 predatory fish or bird that might consider its thin 

 body worth prising oft'. 



The Ark-shells {Area) present an appearance very 

 different from the exceedingly thin Saddle-oysters. 

 The Common Noah's Ark {A. tetragona) is a 

 quaint little yellow and brown box - like shell of 

 distorted aspect that loves to spend its time in the 

 crevices of rocks, or attached by 

 its byssus to the empty shells of 

 other bivalves. A perfect specimen 

 is covered all over with ridges 

 N-h'^ '^' which radiate from the beaks to 



the lower margins, w^hilst other but less prominent 

 ridges cross these at right angles; but owing to 

 its habit of grubbing in crannies and among rubbish, 

 the greater portion of this ornamentation gets rubbed 

 off, as shown in the illustration, and consequently it 

 is not easy to obtain a good example. The beaks are 

 wide apart, and the 40 to 50 hinge-teeth are of simple 

 form. Towards the hinder margin of the whitish 

 mantle there is a number of closely grouped ocelli — 

 dark spots that are not true eyes but which are so 

 sensitive to changes in the intensity of light that 

 they serve the same purpose. They consist of cells 

 filled with a dark pigment and covered with a 

 cuticle of high refractive power. Real eyes are 



