Monotaxis grandoculis vs heterodon???

In identifying some fish species from a recent pacific trip, I ran into this conundrum.  Is Monotaxis heterodon a valid species or is it just a synonym for Monotaxis grandoculis?  In one of my guides, "Micronesian Reef Fishes" by Myers it says the Monotaxis genus is monotypic....however in "Reef Fish Identification: Tropical Pacific" I get both M.grandoculis and M.heterodon.  In looking online I find nothing definitive...if someone could help, that would be great! 

And also if they are indeed two separate species, how to differentiate between the two...thank you!!

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