18 c. M. yon(;e 



carbonate-cemented sandstones, shells of other molluscs, and above 

 all coral rock — and boring is certainly assisted by initial chemical 

 softening of the rock ( Yonge, 1955 ) . 



In both genera the shell is elongated and almost tubular, a modi- 

 fication of the somewhat triangular shell form of the typical mussel. 

 This form, brought about by modification of the components of shell 

 growth (see Yonge, 1955), enables them to penetrate rock with 

 great efficiency. In Botula (Fig. 8) a massive byssus is secreted. 



B 



Fig. 8. Botula falcata. A, boring exposed to show dorsal side of animal, 

 siphons extended and animal withdrawn from head of boring; erosion of shell 

 on either side of hinge region shown. B, boring opened to reveal attachment 

 to "floor" of boring of byssal threads in large anterior and smaller posterior 

 groups, former pulled against when animal bores. C, boring opened at head 

 end. (From Yonge, 1955.) 



So attached to the floor of the boring, contraction of the posterior 

 byssal retractor muscles drives the anterior end of the shell forward 

 against the head of the boring. There can be no question here of 

 water pressure within the mantle cavity because there are no true 

 siphons, but the ligament is long and its opening thrust powerful 

 enough to cause widening of the boring. Although the shell has a 

 thick covering of brown periostracum, this and the underlving 



