There are two extant (living) species:
And two extinct species only known from fossil.1
Bicknell, Russell D. C.; Błażejowski, Błażej; Wings, Oliver; Hitij, Tomaž; Botton, Mark L. (2021-03-02). Zhang, Xi-Guang (ed.). "Critical re-evaluation of Limulidae uncovers limited Limulus diversity". Papers in Palaeontology. 7 (3): 1525–1556. Bibcode:2021PPal....7.1525B. doi:10.1002/spp2.1352. ISSN 2056-2799. S2CID 233783546. https://onlinelibrary.wiley.com/doi/10.1002/spp2.1352 ↩