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It is Oxynoe sp. 1 in IPN at page 67:
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp.
Originally posted at these websites as Oxynoe sp. A Genus: Rafinesque, 1814.
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.
More information on Oxynoe sp. 01 as Oxynoe sp. 4 are on Bill Rudman's Sea-Slug Forum!