They are specimens of Pelycidion matthewsi, Pelycidion cf. matthewsi, Pelycidion cf. megalomastoma and Pelycidion megalomastoma. The first set are from south Florida, but they are also found from (at least) there north to Virginia. The second two sets are from between Wilmington and Myrtle beach, and are found from at least south Florida to southern North Carolina. The final set is from the Caribbean coast of Panama, and they range north to off of South Carolina.
There are no other species in the family known from the northwestern Atlantic, ignoring the two weird undescribed fossil ones that are much less similar to either of those. In the first picture, note the somewhat detached aperture, and lack of any sculpture other than growth lines; in the second set, note the more weakly detached aperture; in the third, the still more weakly detached aperture and the very faint subsutural groove; in the final set, the prominent but not detached aperture and the subsutural groove.
If I had any data for points in between, I could provide them, however, these shells are tiny (thus rarely documented), and most of the deposits from intermediate ages are eroded away in the Carolinas.
What is simpler here? Shifting form over 3.5 million years or separate creations that just so happen to line up with the gaps between all the surviving deposits? Also, if I photographed additional specimens, I could demonstrate a complete blur in form with overlaps between the Pinecrest/Duplin/Yorktown P. matthewsi and the lower Waccamaw specimens; the lower Waccamaw and the upper Waccamaw specimens; and between the upper Waccamaw specimens and the recent shells.
In this case, it is more like “We have four sets of houses. The oldest set was built with circular windows and a large front porch, the next was built using exactly the same types of building materials and blueprints, except for having the front porch average a bit smaller, the next were identical except for a yet smaller front porch and had windows ranging from oblong to square, and the fourth had no front porch or a very small one and consistently square windows, but were otherwise identical to all the previous sets.”
In that case, I think it logical to at least conclude that there was examination of the previous sets of blueprints and instructions, if not the same builders.