Vernacular names: Northern tortoiseshell limpet.
MORE DETAILED ACCOUNT AT https://flic.kr/s/aHskbm8fpS
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Key identification features
1: Maximum length usually 20mm, occasionally 25mm, rarely 30mm.
2: Shell exterior matt-whitish with radiating chocolate-brown rays that often bifurcate and reunite across the shell 1Tt flic.kr/p/WadzLk .
3: Shell interior porcelaneous-white with brown-banded peripheral border, an amphora-shaped, chocolate-brown patch and a pale vertex patch 4Tt flic.kr/p/XbymyT .
4: No prominent sculpture (except sometimes irregular repair-line of damage), but many fine concentric growth lines and radiating striae 5Tt flic.kr/p/XoDw7c .
5: Mantle skirt with emerald green pigment dorsally that looks blue-green when viewed ventrally 17Tt flic.kr/p/X8KpZN . Green is sometimes very pale.
6: Large pallial tentacles protrude beyond shell perimeter when active 23Tt flic.kr/p/Xksgfj .
7: Northern species stretching south to southern Scotland, northern Ireland, Isle of Man, south-west Sweden and Rhode Island, USA.
8: Sometimes, but more often not, on pink, calcareous, encrusting algae with no pale feeding pits, and faecal rods with flat truncated ends that are not chalk-white.
Young Patella spp., can resemble worn specimens of T. testudinalis.
The only ‘tortoiseshell limpet’ in the southern half of Britain where it is often mis-recorded as Testudinalia testudinalis.
1: Maximum length 12mm.
2: Shell exterior whitish/yellowish/bluish with radiating pinkish rays and/or light-brown chains which are often mistaken for brown marks of T. testudinalis, especially small juveniles 42Tt flic.kr/p/XASHRd .
3: Shell interior white often translucent showing exterior marks, sometimes red-brown V near vertex 43Tt flic.kr/p/WCkdUp .
4: Sculpture of slight threadlike radiating striae 43Tt flic.kr/p/WCkdUp , often indistinct or absent 44Tt flic.kr/p/XASHz1 , and numerous fine concentric growth lines
5: Mantle-skirt white, yellowish, or blue-green with reddish bands on periphery 45Tt flic.kr/p/XfRfyN , 46Tt flic.kr/p/WA5eV5 and 47Tt flic.kr/p/XASHvo .
6: Outer edge of mantle has many small, unobtrusive, translucent processes and many large white repugnatorial glands pointing inwards from mantle edge, but no prominent, outward pointing, pallial tentacles 48Tt flic.kr/p/XRRcsx .
7: All round Britain except Liverpool Bay and parts of SE England.
8: Nearly always on pink calcareous encrusting algae with pale feeding pits and short, chalk-white faecal rods with hemispherical ends.
The map provided here shows the distribution of the species based on Conchological Society data.