Hermissenda crassicornis (the more northern species) has vertical white stripes on the front of its cerata (the projections on the back) — see photo above from the Sonoma Coast taken on 10 March 2017.
In contrast, Hermissenda opalescens (the more southern species) lacks those vertical white stripes.
Both species occur in the Bodega Bay region, so you'll need to look closely to tell them apart. Remember, the dominant color of the cerata can vary (e.g., orange, red, or brown), so it's important to check for the presence or absence of the vertical white stripes.
For comparison, here's a picture of what is now called Hermissenda opalescens (photographed on Bodega Head in May 2013):
P.S. If you're curious, the third species, Hermissenda emurai, is found in the western Pacific (e.g., Japan and Russia).