Top: length 13.6mm, height 3.9mm. Apex 36% of length from anterior.
Middle: length 18.4mm, height 4.5mm. Apex 34% of length from anterior.
Bottom: length 23.9mm, height 7.3mm. Apex 37% of length from anterior.
In Britain, sublittoral specimens usually grow up to 20mm long, 14mm wide, 10mm high, but 15mm is usual maximum length of intertidal specimens. The bottom specimen in this image exceeds both these maxima. 30mm long specimens are sometimes found in N. America.
Usually, apex is closer to centre (about 25% to 40% of length from anterior) than on Tectura virginea (about 18% to 30% of length from anterior), but both species vary and ranges overlap.

APPENDIX re range advance/retreat: flic.kr/p/2jW74ig

marinvert.senckenberg.science/image-browse/

Key identification features
Testudinalia testudinalis
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 01 Testudinalia testudinalis. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. .
3: Shell interior porcelaneous-white with brown-banded peripheral border, an amphora-shaped, chocolate-brown patch and a pale vertex patch 04 Testudinalia testudinalis. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. .
4: No prominent sculpture (except sometimes irregular repair-line of damage), but many fine concentric growth lines and radiating striae 05 Testudinalia testudinalis. Length 12.1mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. .
5: Mantle skirt with emerald green pigment dorsally that looks blue-green when viewed ventrally 17 Testudinalia testudinalis. Shell length 14.1mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. . Green is sometimes very pale.
6: Large pallial tentacles protrude beyond shell perimeter when active 23 Testudinalia testudinalis. Shell length 13.2mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. .
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.

Similar species
Young Patella spp., can resemble worn specimens of T. testudinalis.

Tectura virginea
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 42 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. Shell lengths 6mm & 9mm. North Yorkshire and Menai Strait. .
3: Shell interior white often translucent showing exterior marks, sometimes red-brown V near vertex 43 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. Shell length 7mm. Oban, Scotland. August 1969. .
4: Sculpture of slight threadlike radiating striae 43 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. Shell length 7mm. Oban, Scotland. August 1969. , often indistinct or absent 44 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. Shell-length 6mm. From top down: anterior, posterior and lateral views. North Yorkshire. September 2014. , and numerous fine concentric growth lines
5: Mantle-skirt white, yellowish, or blue-green with reddish bands on periphery 45 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. 5.5mm long. West Anglesey, Wales. March 2014. , 46 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. 6.6mm long. Yorkshire, England. September 2014. and 47 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. 8mm long. Menai Strait, Wales. March 2012. .
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 48 Testudinalia testudinalis album. COMPARISON image of Tectura virginea. Shell length 10.5mm. West Anglesey, Wales. March 2014. .
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.