Disarticulated valves, tail and head valves enlarged. Sublittoral, Orkney. February 2015. Leg. P. Lightfoot per J. Porter.
Eight overlapping valves have a lower layer, ‘articulamentum’, of white aragonite (sometimes jugal tract is pink 04 Tonicella marmorea or yellow 05 Tonicella marmorea ). On valves ii – viii it projects forwards as a pair of curved, wide, short (front to rear) apophyses (1) separated by a narrow, straight, jugal sinus (gap) (2) that is less than 25% width of an apophysis. At each end of a valve a pair of lateral muscles, one on each side of the slit, connect to the foot, leaving an opaque scar when removed (3).
On each side of a valve an oblique muscle attaches to the jugal sinus and travels anteriorly outwards . The anterior end of the muscle attaches to the body wall under the preceding valve on which it leaves a comma shaped scar (4).

marinvert.senckenberg.science/image-browse/
Revised, 2020, PDF version at www.researchgate.net/profile/Ian_Smith19/research

Key identification features
There is interspecific overlap of many features. The most reliable are valve and girdle surface sculpturing.
Tonicella marmorea O. Fabricius, 1780.
Following is for NE Atlantic. Those in NW Atlantic may differ.
1. Big species. Max. length in NE Atlantic 45mm.
2. Colour variable; dirty white, cream, various shades of brown, red, and ochre arranged in blotches or intricate zig-zag lines. Valves ii, iv and vii often have more dark colour than the other valves 09 Tonicella marmorea .
3. Girdle is yellowish with a brown or purplish band at each end of valves ii to vii, and several bands by valves I and viii. Usually several yellowish blotches on bands 07 Tonicella marmorea . Frequently, part or all suffused with verdigris-green 40 Tonicella marmorea .
4. Apart from growth lines, dorsal surface of valves appears smooth to naked eye 06 Tonicella marmorea , but at over X3 (hand lens) magnification, numerous distinct small granules visible 18 Tonicella marmorea . (Granules may not be discernible in underwater images or on specimens before deposits and properiostracum removed.)
5. Dorsal surface of girdle (perinotum) has widely spaced minute granules (c.27μm long) 40 Tonicella marmorea . Usually escape notice in photographs; require strong magnification.
6. At LWS and sublittorally. Not found south of North Wales or Northumberland. (Many dubious records on mapping schemes as easily confused with T. rubra.)
7. 17 – 26 ctenidia along most of each side of foot, arrangement merobranch but nearly holobranch 46 Tonicella marmorea .
8. White apophyses on intermediate valves are short (anterior-posterior), wide and gently curved 28 Tonicella marmorea . Narrow jugal sinus (gap between them) is about quarter width of one apophysis.

Similar species

Boreochiton ruber (Linnaeus, 1767) (Formerly T. rubra)
1. Medium species. Max. length 15mm.
2. Dorsal surface of valves pink to brick red with patches and streaks of cream and light brown 60 Tonicella marmorea COMPARISON Boreochiton ruber .
3. Girdle usually has reddish/pinkish bands alternating with pale bands. Pale bands, often fragmentary, usually thinner than reddish/pinkish bands, and roughly in line with shell-sutures 60 Tonicella marmorea COMPARISON Boreochiton ruber .
4. Apart from growth lines, dorsal surface of valves appears smooth to naked eye, but at X30 magnification (dissecting microscope). faint stippling is visible 61 Tonicella marmorea COMPARISON Boreochiton ruber .
5. Dorsal surface of girdle (perinotum) has densely-packed, elongate (50 – 60μm), club-shaped granules with packed touching heads giving a mealy appearance. 62 Tonicella marmorea COMPARISON Boreochiton ruber .
6. At LWS and sublittorally. All around Britain except NE Irish Sea and southern North Sea where hard substrate is scarce.
7. About 12 ctenidia each side (range 10-15), arrangement merobranch.
8. White apophyses on intermediate valves are prominent and rounded, often almost semicircular 61 Tonicella marmorea COMPARISON Boreochiton ruber . Wide jugal sinus (gap between them) is about same width as one apophysis.

Lepidochitona cinerea (Linnaeus, 1767).
1. Medium species. Max. length 28mm.
2. Has diverse colour forms, some similar to that of other species 63 Tonicella marmorea COMPARISON Lepidochitona cinerea .
3. Girdle usually has alternating brown (various shades) and whitish, transverse, lozenge-like bands of approximately equal size (diagnostic in NE Atlantic). Dark bands usually have narrow waist, and light bands usually have a bulging waist 64 Tonicella marmorea COMPARISON Lepidochitona cinerea (or vice versa). A thin paler greyish longitudinal line often runs across the waist of dark bands. Usually a dark central spot on pale bands. Markings can be partially developed, very indistinct, or absent 63 Tonicella marmorea COMPARISON Lepidochitona cinerea on pale specimens.
4.Dorsal surface of valves has numerous rounded granules visible at X3 magnification, and often to naked eye. Granules tend to follow indistinct growth lines 65 Tonicella marmorea COMPARISON Lepidochitona cinerea .
5. Dorsal surface of girdle has densely packed rounded granules 65 Tonicella marmorea COMPARISON Lepidochitona cinerea .
6. Commonest littoral chiton all around Britain. Only chiton species likely to be found higher than MLW on British shores.
7. Usually 16 – 19 ctenidia each side, arrangement almost holobranch 66 Tonicella marmorea COMPARISON Lepidochitona cinerea .
8. White apophyses on intermediate valves are gently curved. Wide convex jugal sinus (gap between them) on valve iv is about same width as one apophysis 6 67 Tonicella marmorea COMPARISON Lepidochitona cinerea .

Callochiton septemvalvis (Montagu, 1803) (Formerly Callochiton achatinus
1. Big species. Max. length 32 mm.
2. Dorsal surface of valves usually some shade of red, sometimes yellow or green, with or without irregular blotches or marbling. Head and tail valves, i and viii, are often darker than the other valves 68 Tonicella marmorea COMPARISON Callochiton septemvalvis . Minute black spot (X30 to see on pale area of valve) by each apical cap of aesthete canals on valves i and viii, and on lateral triangle of intermediate valves ii to vii 69 Tonicella marmorea COMPARISON Callochiton septemvalvis .
3. Girdle colour is usually similar to that of adjacent valve, often with a transverse, diagonal, cream band by sutures i/ii and vii/viii 70 Tonicella marmorea COMPARISON Callochiton septemvalvis , and occasionally other sutures 68 Tonicella marmorea COMPARISON Callochiton septemvalvis .
4.Dorsal surface of valves has numerous grooves that give a granular appearance visible at X8 magnification 69 Tonicella marmorea COMPARISON Callochiton septemvalvis , but often not visible in photographs.
5. Dorsal surface of girdle has densely packed, tessellated, 200μm long, acicular spicules that create a characteristic snake-skin appearance 71 Tonicella marmorea COMPARISON Callochiton septemvalvis .
6. Rarely numerous. South, west and north coasts of Britain. Scarce on east coasts; very scarce or absent in southern North Sea and north-east Irish Sea.
7. Usually 20 to 25 ctenidia each side, arrangement merobranch.
8. White apophyses on intermediate valves are short (anterior-posterior), wide and gently curved 7 72 Tonicella marmorea COMPARISON Callochiton septemvalvis . They meet at the jugum with no jugal sinus (gap) between them. Only British species with two slits and slit rays (sometimes a small third one) in the articulamentum on either side of the imtermediate valves, ii to vii.