If I'm identifying these correctly, this was by far the most abundant small ark shell--the most abundant shell in fact--on most of the beaches we visited. Literally millions of them could be seen in the high tide drift lines at any beach. The process of shelling was often just scanning, sorting, and sifting through masses of these to look for other species. See, for example, this earlier upload of a typical beach scene:
http://www.inaturalist.org/observations/2277245
These typically ranged in size from 3/4" to 1" in length. I'm still working through our collection of ark shells here at home, hoping I might discover one or more additional ark species for our list.