Genera in the largest family of the suborder Sacoglossa (Mollusca; Opisthobranchia), the Plakobranchidae Gray, 1840, have been systematically revised numerous times since the 1800s. Several authors have questioned the validity and inter-relationships of the genera Tridachia Moerch, 1863, Tridachiella MacFarland, 1924, Elysiella Bergh, 1872, Pattydaya Marcus, 1982, Elysia Risso, 1818, and Thuridilla Bergh, 1872. For many other groups, molecular data have proven fruitful in determining the

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... tic relationships of organisms for which few suitable morphological characters are available. Using DNA sequence data from one nuclear (Histone 3) and two mitochondrial genes (Cytochrome Oxidase subunit I and large ribosomal subunit), we infer the phylogenetic relationships among five of the seven recently recognized genera within the Plakobranchidae. These data question the monophyly of the genus Elysia and suggest that further divisions within the genus may be necessary. Additionally, it appears that the family Boselliidae Marcus, 1982 may be, at least, paraphyletic since one member of the genus Bosellia Trinchese, 1891, clusters within the family Plakobranchidae.