DEMINERALIZATION MECHANISM OF BORING GASTROPODS 85 



disfigure this uniformity. Whether such regions contain greater con- 

 centrations of conchiohn which slow penetration of the ABO secre- 

 tion, or interfere with rasping, is not yet known. The fact that shell 

 softening occurs chiefly at the bottom of the hole and that the radula 

 rasps principally also at this suiface accounts for the fact that the 

 muricid bore hole has relatively straight sides (Figs. 1 and 2). That 

 some secretion also penetrates the walls of the hole is strongly sug- 

 gested bv the microradiographs (Figs. 5 and 6). The presence in 

 most of these perforations of a very slight bevel at the external 

 surface of the shell suggests that some of the ABO secretion spreads 

 beyond the hole under the fcot, permitting the radula to remove 

 shell there too. 



Because of the closeness of the propodial folds around the func- 

 tioning naticid proboscis tip and appended ABO, it is unlikely that 

 much, if any, dilution of the ABO secretion occurs when the pro- 

 boscis is everted. The naticid ABO, though at least as efficient as 

 the muricid ABO in softening shell, differs from the latter in being 

 a padlike organ which produces a broadly beveled hole (Figs. 3 

 and 4), probably because it cannot be extended as precisely and 

 fully into the concavity as the muricid ABO. In this sense naticid 

 boring is less economical than that of muricids, since more shell 

 must be removed to effect an opening of a given diameter at the 

 bottom of the hole. 



The action of the ABO secretion of different species of boring 

 gastropods tested to date appears to be similar, not only on a given 

 species of mollusk shell, but on the different species employed. The 

 variation in ultrastructural topography observed on different etch- 

 ings is a reflection of the degree of alteration of the shell rather 

 than of species differences in the ABO or the shell substrate. 



Figs. 44 to 49. Effects of secretion of excised ABO of Urosalpinx cincrea 

 follyensis on polished surfaces of ground sections of human teeth exposed to 

 3 successive periods of ABO activity totaling 60 hours. ( X 6000.) 



Figs. 44 and 45. Sharp demarcation between affected and normal enamel. 

 Figs. 46 and 47. Progressively increasing damage to enamel. 

 Fig. 48. Normal dentin. 



Fig. 49. Surface of dentin affected by ABO secretion; the more pro- 

 nounced tubular, peritubular, and intertubular structures are evident. 



