Strongylocentrotus franciscanus and Strongylocentrotus nudus are referred to a new genus Mesocentrotus within the Strongylocentridae based on DNA-DNA hybridization patterns. Additional data on the morphology of the Larval skeleton and Tooth plates of these urchins and, Pseudocentrotus depressus, Lytechinus variegatus, Sphaerechinus granularis and Tripneustes ventricosus is presented.