There are four recognized species of Herpetocetus: H. scaldiensis,H. transatlanticus,H. bramblei, and H. morrowi. An additional unnamed species from Japan has been referred to the nomen dubiumMizuhoptera sendaicus in the past, but the undiagnostic nature of the holotype of the latter means that it cannot be confidently referred to M. sendaicus.
Herpetocetus and other Half Moon Bay genera
Survival into the Pleistocene
A recently discovered specimen of Herpetocetus from the Lower-Middle Pleistocene (Gelasian) Falor Formation of northern California indicates that Herpetocetus survived into the early Pleistocene, shedding light on the diversity of mysticetes in the early Pleistocene. Since other cetotheriids became extinct by the end of the Neogene, it's therefore possible that some Pliocene representatives of Cetotheriidae made it into the Pleistocene to co-exist with extant mysticete species.
^P. J. Van Beneden. 1872. Les Baleines fossiles d'Anvers. Bulletins de L'Academie Royale des Sciences, des Lettres et des Beaux-arts 34:6-23
^ abWhitmore, F.C., and L.G. Barnes. 2008. The Herpetocetinae, a new subfamily of extinct baleen whales (Mammalia, Cetacea, Cetotheriidae). In C.E. Ray, D.J. Bohaska, I.A. Koretsky, L.W. Ward, and L.G. Barnes (eds.). Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virginia Museum of Natural History Special Publication 14:141-180.