Cornulites worm tubes are something I've found before attached to brachiopod shells. Generally they are small, under a cm in length, and have widely (for the scale) spaced growth lines. The specimen below I purchased from a dealer and is much, much larger than any I have seen before.
The overall length of the specimen is about 2.5 inches or 6 cm.
The growth lines are much more closely spaced than other specimens I have seen. This could just be related to the species of worm or possibly the diameter of the tube (about 1/2in or 1cm).
The specimen came from the Waldron shale in Indiana which is Silurian (Sheinwoodian to Homerian stage) in age.