The belemnicolid borings Dendrina, regarded previously as attributable to parasites of the belemnites, or to organisms benefiting from belemnite carrion, are interpreted as left by hard-substrate borers inhabiting loose guards lying on the sea bottom. The boring activity was herein confined to these parts of guards which projected above the bottom. With regard to taphonomical analysis, a conclusion arises that the boring activity may be presumably attributed to primitive algae.