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This species has a very strange distribution. It was described by Trinchese in 1893 as Hermaea cremoniana from Naples in the Mediterranean Sea in:
Trinchese, S. (1893) Nuovi Ascoglossi del golfo di Napoli. Rendiconti dell'Accademia delle Scienze Fisiche e Matematiche della Societa di Napoli, 2(7): 154.
and then Baba (1959) reported it from Japan in:
Baba, K. (1959) The family Stiligeridae from Japan (Opisthobranchia- Sacoglossa). Publications of the Seto Marine Biological Laboratory, 7: 327-334.
Since then it has been found at various other places in the western Pacific as far south as Sydney in eastern Australia, in Hawaii, and in the Indian Ocean at eastern Africa, Kenya.
More information on Placida cremoniana are on Bill Rudman's Sea-Slug Forum!