An Elf's Cap and an "Oyster"

Click on images to enlarge, found in Benton county, Arkansas, Boone limestone formation

Grey fragments of mollusk shells can often be found in the crinoid-rich limestone of Benton county, Arkansas. This is a nicely intact specimen dug from the matrix. Note the parasitic borings on the enlarged version of the picture.

This elf's cap-shaped critter has a tail that reminds me of a brachiopod, but it is elongated too much to be one, IMOH. Any ideas? More pics in this article. Update: Thanks to visitor Barry for identifying both of these specimens as Platyceras species!