1: slight threadlike radiating striae.
2: fine concentric growth lines emphasised by algal growth.

KEY ID FEATURES BELOW

marinvert.senckenberg.science/image-browse/

Similar species
Testudinalia testudinalis (O. F. Müller, 1776)
1) Maximum length usually 20 mm, occasionally 25 mm 31Tv 31 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Kincardineshire, Scotland. .
2) Shell exterior matt-whitish with radiating dark brown rays that often bifurcate and reunite across the shell 30Tv 30 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Banffshire, Scotland. Leg. S.Taylor. .
3) Shell interior porcelaneous-white with brown-banded peripheral border, and dark brown viscera patch, usually with pale vertex patch 33Tv 33 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Banffshire, Scotland. Leg. S.Taylor. .
4) Fine concentric growth lines and radiating striae 32Tv 32 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Banffshire, Scotland. Leg. S.Taylor. .
5) Mantle skirt green in pallial groove, no red bands or large white glands on mantle periphery. Ctenidium, no pallial gills 34Tv 34 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Banffshire, Scotland. Leg. S.Taylor. .
6) Large pallial tentacles protrude beyond shell perimeter when active 34Tv 34 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Banffshire, Scotland. Leg. S.Taylor. .
7) North U.K, to Anglesey and N. Yorkshire. Many absence records, misleadingly, mapped further south on NBN with same symbol as for presence. On encrusted rock, not on seaweeds. records.nbnatlas.org/occurrences/search?q=lsid:NHMSYS0021…
8) Apex c. 25% to 40% of length from anterior 32Tv 32 Tectura virginea album. COMPARISON image of Testudinalia testudinalis. Banffshire, Scotland. Leg. S.Taylor. .

Patella pellucida Linnaeus, 1758 [early stage length 2 to 7mm]
1) Maximum length of development stage 7 mm.
2) Shell exterior translucent pale horn to light brown with blue/green hyphens in longitudinal rows 36Tv 36 Tectura virginea album. COMPARISON image of Patella pellucida. Anglesey, Wales. . not in links of a chain pattern like that on some young T. virginea 3Tv flic.kr/p/tg3WuB .
3) Shell interior pale horn to light brown with black, colloidal particles under parts of the blue/green hyphens.
4) Smooth fragile shell 36Tv 36 Tectura virginea album. COMPARISON image of Patella pellucida. Anglesey, Wales. .
5) Mantle-skirt translucent whitish. Pallial gills, no ctenidium in nuchal cavity. No inwardly oriented, large, white repugnatorial glands or reddish bands on mantle periphery 35Tv 35 Tectura virginea album. COMPARISON image of Patella pellucida. Anglesey, Wales.
6) pallial tentacles protrude from aperture rim, but unobtrusive as translucent and almost invisible 35Tv 35 Tectura virginea album. COMPARISON image of Patella pellucida. Anglesey, Wales. .
7) All round Britain except Liverpool Bay and parts of SE England. On Laminaria, not rock, when > 2mm long.
8) Apex lost from anterior rim of aperture 36Tv 36 Tectura virginea album. COMPARISON image of Patella pellucida. Anglesey, Wales. .

Williamia gussoni (Costa O. G., 1829) 37Tv 37 Tectura virginea album. COMPARISON image of Williamia gussoni . Shell length 8.8 mm. Posterior on right. Croatia. © P. Ugarković.
1) Usual shell length c. 6 mm. Maximum L. c. 8 mm, W. 6 mm and H. 3 mm (Ruthensteiner, 2006); L. 8.8 mm (P. Ugarković, 2020 pers. comm.)
2) Exterior of shell is shiny, bright red-brown with c. 18 pale radiating rays that are sometimes obscured or faded when dead. Periostracum extends well beyond rim of shell.
3) Shell interior as exterior, but paler.
4) Thin, smooth shell with convex anterior and posterior slopes.
5) Mantle is translucent, red-brown with radiating, opaque white rays which correlate with pale rays of shell. No inwardly oriented, large, white repugnatorial glands. In Siphonariidae; respires with c. 17 lamellae concealed in mantle cavity; no pallial gills or protruding ctenidium plume.
6) No pallial tentacles. Cephalic tentacular lobes separated from large flat head by small cleft. Large flap-like anal lobe protrudes from mantle cavity on right. Dorsal surface of head and foot are red-brown; ventrally yellowish white
7) Common at Low Water to more than 50 m (J. Prkić, 2020, pers. comm.) in Adriatic, Mediterranean and adjacent Atlantic; not N.W. Europe (Ruthensteiner, 2006).
8) Apex has distinct semi-spiral protoconch positioned off-centre posteriorly, sometimes overhanging beyond the posterior of the aperture.