Previous works discussing interrelationships of squamates, mosasauroids, and aigialosaurs have alternately suggested Opetiosaurus bucchichi and Aigialosaurus dalmaticus to be accurately diagnosed, congeneric, and in some cases, conspecific taxa. Recent systematic work has shown that these two species, as basal mosasauroids, are critical to understanding the evolutionary history of mosasaurs. Because earlier attempts at descriptions of O. bucchichi have proven insufficient for systematic purposes, the specimen is redescribed here based on the counterpart skull to the holotype part specimen, revealing new anatomical details unavailable to previous workers. This comparison of O. bucchichi and A. dalmaticus reveals no morphological differences that support a generic differentiation of the two specimens assigned to these monospecific genera. The genus Opetiosaurus is therefore a junior synonym of Aigialosaurus and O. bucchichi is re-diagnosed as A. bucchichi; the species A. bucchichi is retained because some minor differences are present between the individual specimens assigned to the respective species. Incorporation of new anatomical information into a phylogenetic analysis lends support to suggestions from previous workers that the paddle-shaped limb evolved more than once within Mosasauroidea; however, the tree topologies are not robust. The results of this study indicate that the systematic relationships of halisaurine mosasaurs are of primary importance in the study of mosasauroid interrelationships.