>...or are there other gastropods around which bore cylindrical holes? (don't believe so). Countersunk naticid borings are far more common there, and as yet I have seen no Octopus borings... but would really like to... direct evidence of Oligocene octopuses!<

Unfortunately, the straight hole=muricoid/countersunk hole=naticid principle is an overgeneralization that does not seem to hold up on closer examination.  Some muricid species make nicely countersunk holes.

At least some marginellids can bore shells, but I do not remember what the hole shape is.

Occasionally, it may be possible to identify a likely source for fossil predatory borings.  Pliocene large Mercenaria from the eastern U.S. sometimes have a large straight hole through them.  The only large boring predator around is Ecphora.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa