Meet Hancockia californica. This individual was spotted at Coleman Beach on 23 May 2015. It was ~15 mm long.
I mentioned Hancockia is different than many other nudibranchs. This is true in several ways, one of which includes the shape of the cerata (the upright projections on its back). The cerata are palmate — each is made up of 4-16 finger-like processes in a U-shaped cluster.
The illustrations of Hancockia in MacFarland's 1923 paper makes this easier to see:
From MacFarland, F.M. 1923. The morphology of the nudibranch genus Hancockia. Journal of Morphology 38: 65-104. (Those page numbers are correct — this is an extensive article!)
Now that you know what to look for, here's a close-up of these unusual cerata:
When reading MacFarland's article, I was glad to see a couple of other things mentioned that stood out to us when watching this nudibranch.
In MacFarland's words, Hancockia has a "prominent cardiac elevation." Translation: the position of its heart stands out because of a raised bump on its back — this bump was so "elevated" that it looked like a small bubble. In the picture below, it's the rounded area with squiggly red lines:
MacFarland also commented on Hancockia's unusual way of moving:
"In captivity it is at first very active, continually creeping around the sides of the aquarium and upon algae by a method entirely different from the smooth gliding movement of other Nudibranchs. The anterior third of the body, i.e. the portion in front of the heart region and the second pair of cerata, is stretched forward to its full extent, the anterior end of the foot and the lip region are closely applied to the surface and adhere tightly while the rest of the body is pulled forward, a distance of 1 or 2 mm at a time, the posterior end of the body taking but little active part in the process."
Translation: Hancockia moves like an inchworm!
Here's a short video clip so you can see (1) the beating heart, (2) the obvious ridge zigzagging down its back ("vascular ramifications to the cerata" = basically a raised conduit bringing blood from the heart to the cerata), (3) the unusual palmate cerata, and (4) a hint of the inchworm-like movement:
P.S. If you were wondering, the genus Hancockia was named after the English naturalist Albany Hancock, co-author and primary illustrator of "A Monograph of the British Nudibranchiate Mollusca: with Figures of All the Species" published in eight parts by The Ray Society between 1845-1855.