DEMINERALIZATION MECHANISM OF BOEING GASTROPODS 71 



shell surface in the bore hole with unaffected mineral-conchiolin 

 shell surface around it. To obviate this difficulty we removed the 

 periostracum and polished the external surface of shell of living 

 oysters with a small electric sander and buffer. Snails readily bored 

 into these oysters, but detritus still interfered with comparisons of 

 shell surfaces; in addition, this experimental approach did not per- 

 mit a clear distinction to be made between effects of etching by 

 the ABO secretion and rasping by the radular denticles. 



Because of the roughness of the external surface of most bivalve 

 shells, and particularly those of ovsters; because of the fineness of 

 etching by the ABO and the difficulty of studying this on rough sur- 

 faces; and because to date it has not been possible to stimulate live 

 snails to bore shell unless there is living prey within, it was necessary 

 next to employ excised ABOs, disk down, on the smooth nacreous 

 inner surface of mollusk shell. As described in the section on methods, 

 we also subsequentlv used excised ABOs enclosed in cylinders of 

 minute tubing, and drops of diluted ABO homogenate. Because bor- 

 ing gastropods appear to drill with equal ease through both nacreous 

 and prismatic portions of calcitic and aragonitic mollusk shells, the 

 inner surface was considered a representative substrate for these 

 studies. Typical etchings produced by these means on different kinds 

 of shells are shown in Figs. 7, 8, 16, and 18 at low optical magnifica- 

 tions. 



Electron microscopic examinations were made of unaffected areas 

 near the region on which the ABO had been placed and of the etched 

 areas under the ABO. As illustrated in Figs. 9 and 10, the normal 

 aragonitic nacreous surfaces of Dosinia discus and Murex fulvescens 

 appeared quite smooth, whereas on the calcite nacreous surface of 

 Crassostrea virginica (Fig. 11) very definite crystal outlines, similar 



Fig. 9. Electron micrograph made from palladium-shaclowed positive col- 

 lodion-carbon replica of normal nacreous surface of Dosinia discus, showing 

 a relatively smooth siuface. ( X 6000. ) 



Fig. 10. A similar micrograph of the normal nacreous surface of Murex 

 fulvescens, showing very faint structural detail, (x 6000.) 



Fig. 11. A similar micrograph of the normal nacreous surface of Cras- 

 sostrea virginica, illustrating the characteristic and markedly crystalline nature 

 of the surface. ( X 6000.) 



