Found these on the same piece of drift kelp as Triopha maculata. I've included a photo of the carina which is laterally expanded like whale flukes, and distinguishes this species from Lepas anatifera.
43mm length. Saw 4 of these guys, all on the same piece of drift kelp.
29mm length. Went to Monterey Harbor yesterday and found only 3 Hermissenda, no other nudibranchs; then went to Elkhorn Yacht Club and saw 3 of these guys on Bugula with many egg masses.