8 



retreat iii<>' far iiitu the siiell, the part of the animal 

 nearest the opening being' further proteeted in manj^ eases 

 by an opercrihim. When thus retraeted, the animal can 

 be knocked about without incurring serious risks. This 

 form of protection is suited to an actively creeping- 

 animal. In a cap-shelled form the power of retraction is 

 much less and the operculum has gone, so the animal 

 draws doAvu its cap and remains tightly affixed to the 

 spot until the danger has passed. 



Internally the shell shows markings, of which the most 

 conspicuous is the horse-shoe shaped impression, with 

 expanded front ends, of the shell muscle. This impres- 

 sion is divided into partially distinct areas corresponding 

 to the component bundles or fasciculi making up this 

 muscle. Un the outer side of the horse-shoe muscle we 

 see the pallial impression Avhich is continuous across the 

 front of the shell. 



Sections of the shell show its structure to be as follows 

 (Plate L, fig. 2): — 



A. — A thin irregular external layer of brownish colour. 



13. — A middle layer made up of very compact crystalline 

 material the external part of which (a) is penetrated in 

 every direction by branching minute canals while its 

 inner part (h) is comparatively clear. 



C. — An internal layer made up of numerous smaller 

 layers extending for varying distances. Each such layer 

 is, in its turn, made up of parallel but obliquely arranged 

 lamelhe. This portion is thickest under the dome. It 

 is the nacreous or pearly layer. 



Accounts of the shell structure have been given by 

 Gibson and by Boutan. Gibson noted the minute 

 branching canals in the external part of the shell and 

 ventured to suggest that they were due to boring Alga?, 

 which our observations make us think highly probable. 



