These beads are probably porcelain, or at least a white ceramic. I would have to guess they are Central European rather than Venetian. (I could not exclude France as a possible maker—or any European nation that produced ceramic wares.) As Rosanna mentions, it is possible to see some traces of the now-eroded painted decorations that made them resemble chevron beads. (See the screenshot below, posted by Thomas Stricker, where five specimens are shown.)

In the early days of The Bead Journal (beginning in 1974), Robert Liu occasionally composed articles about porcelain trade beads—many of which were copies of other beads, including native-made beads. Copies of mollusk shell artifacts were typical objects.

But few authors (that I am aware of) have continued to observe and discuss these beads. One exception is the report on the Bapterosses industry at Briare, composed by the Picards. See the screenshot below, where white porcelain imitations are mentioned—and follow the link to read this article. But whether Bapterosses made the beads in-question here remains unresolved.

JDA.