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Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books. Gig Harbor, Washington. 426pp. at page 77,
let me quote:"This species can be recognized by the opaque white pigment on the head and rhinophores and the black oval area situated immediately behind the rhinophores."
This picture shows the distinctive eye spots of the genus Costasiella: relatively close together between the rhinophores.
Originally posted at these websites as Costasiella sp. A Genus: Pruvot-Fol, 1951.
For some genera, such as Trinchesia is the alphabet almost through. I had to change the distinguishing alphabetic character into a number. I have chosen 2-digit numbers, even though '01' looks a little bit stupid, and now is only the genus italic written.