Siliceous scales resembling those of the living haptophyte Hyalolithus neolepis Yoshida et al. were discovered in middle Eocene outcrop sediments from the Kellogg Shale in California, and Chalky Mount and Springfield in Barbados. Like H. neolepis, the fossil scales have a marginal rim, hyaline margin and numerous openings in the central area. However, they differ in the nature of the pit-like depression on the distal surface and the corresponding swelling on the proximal surface. Such swellings are very rare in H. neolepis and were not part of the original description. The presence of a swelling on all fossil scales found so far is sufficient enough to warrant the erection of H. tumescens Abe, Tsutsui & Jordan sp. nov. These findings represent the oldest known fossil record of the Prymnesiales.