Unfortunately the shell, which is otherwise in excellent condition, has a fragment of a different shell stuck lengthwise in its aperture.
Siphopteron sp. RB1 of Robert Burn, Nudibranchs and Related Molluscs. Found by Audrey Falconer. @audreyfalconer1
This observation is part of a long-term project examining plankton biodiversity in the nearshore waters around the Hakai Institute's Quadra Island ecological observatory. A COI DNA sequence from the organism has been deposited in the Barcode of Life Data System (BOLD). Photo license and credit belong to the Hakai Institute.