Found in a Rockpool by Students on SU Marine Zoology Fieldcourse
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(I'm sorry the Marlin site I quoted h.b. out of action). My Collins (Hayward etc) says same "..leaving the mid dorsal region uncovered..", but there are plenty of good ex's in google images where it doesn't show due to length of ceratas, & EOL, http://eol.org/pages/402852/media has plenty more.
Again on the V feature, plenty of google images of A.p. show fronts without any marks, & Habitas, http://www.habitas.org.uk/marinelife/species.asp?item=w15510 says "there is usually a V-shaped...", though EOL is categorical.
I couldn't find any pics quite as chunky as this nor with the remarkable colour (but the Marlin ex. gets near), & so a great shot & i.m.o. the best A.p. on Ispot
Thanks for the comments. Looks like its a bit of a tricky one, might even be a subadult (juvenile)?. I should have whipped its bits out. Hope to see some more on next years trip. Makes a great screen wallpaper.
Lat/Lng: 55.7914, -4.9113
OS grid ref: NS175591