ROCK-BORING ORGANISMS 19 



calcareous laver are invariably deeply eroded around the umbones 

 ( Fig. 8A ) . Boring is straight into the rock, with a pronounced dorsal 

 and a smaller ventral ridge (Fig. 8C). B. californiensis and B. 

 falcata are both common borers, largely between tidemarks, in soft 

 rocks along the coasts of central California where thev were ex- 

 amined ( Yonge, 1955 ) . 



Lithophaga (Lithodomus) lithophaga, the date mussel of the 

 Mediterranean, has been known since classic times. There are many 

 other species in warm-temperate and tropical seas, and thev are 

 particularly common on coral reefs, five species occurring on Low 

 Isles, Great Barrier Reef (Otter, 1937). Compared with Botula, the 

 shell is more completely rounded in section and is everywhere 

 covered by a particularly thick and dark-colored periostracum. This 

 is never eroded. In most species of Lithophaga anterior and pos- 

 terior areas of the shell are covered with a granular calcareous 

 deposit (Fig. 9). Although bvssal attachment persists throughout 







a b c d 



Fig. 9. Lithophaga cumingiana. Above, animal //) situ, in boring; below, 

 transverse sections in regions a to d. Small extent of byssal attachment shown, 

 also adherent calcareous debris over anterior and posterior regions of shell 

 (but no erosion). Calcareous lining of boring (as in Rocellaria) secreted by 

 walls of siphons, indicated in section by thick line, marginall)- (ventrally and 

 anteriorly) by broken line. (After Otter, 1937.) 



