Tail valve viii from specimen 8.6 mm long.
Anterior of valve on the right of both images.
Left image; tilted down at anterior to expose full surface.
Right image; in approximately life position as for following descriptions:

Tail valve roughly triangular; mucro (X) behind the centre when seen from above, swollen but not prominent ….. (Kaas & Belle, 1985).
Length of tail valve 45% of width (Kaas, 1981).
Llŷn, North Wales. March 2016.
The illustrated specimen (right image) differs from the quoted descriptions as the outline is almost semicircular, and the length of the valve is 51% of the width (almost midway between figures in Kaas for L. cancellatus, 45%, and L. sarsi, 60%). Note that the posterior position of the mucro applies only to an extracted valve as viewed on the right; on live specimens the white unstained anterior border visible in this image is concealed below the next valve resulting in an apparent anterior mucro position. See image 7 07 Leptochiton cancellatus
The left image, because of the tilted view, shows greater deviation; mucro to anterior of centre, and length 68% of width, so this view should not be used for identification.

KEY ID FEATURES and similar species at: 02 Leptochiton cancellatus