| Guard papillae around inhalant opening | Non-branching papillae; few branching near exhalant si- phon and mostly non-branching papillae (total 9 papillae on | Simple, small | Simple, large, increasing in size towards exhalant si- phon; | Simple, small, numerous (≤ 14 papillae) | Branching papillae; simple or branched, small and nu- |
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| each side on inhalant siphon); 9–12 simple or branching pa- pillae that are largest midway between exhalant siphon and ventral extremity of inhalant siphon | | 4–12 simple or branching papillae that are largest midway between exhalant siphon and ventral ex- tremity of inhalant siphon | | merous (12 in SL = 7.7 mm individual) |
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| Colour of inhalant and exhalant regions | Dark yellow with opaque white spots | Fine opaque pinkish spots on pale or slightly brownish background | With fine opaque white spots | Fine, opaque white spots | Yellowish brown |
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| Size of labial palps | A few folds per palp;16 folds per palp in 10 mm animal; between 9 and 16 folds per palp for SL range 8.0–12.5 mm animals (see Fig. 11) | Medium size, 20 folds per palp in 7 mm animal; outer palps generally larger than inner palps; number of folds vari- able (40 folds in 12 mm animal; see Fig. 11) | Large, 1/3 length of gills; 31 folds per palp in 7 mm animal; outer palps gen- erally larger than inner palps; number of folds variable (57 folds in 12 mm animal; see Fig. 11) | Short, one-fifth length of gills, 18 folds per palp in 12 mm animal (see Fig. 11) | Short, one-fifth length of gills, 15 folds per palp in 7.7 mm individual |
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| Size of anterior | Small and simple; condensed | Branching, deeply pene- | Condensed mass above base | Condensed mass | Condensed mass |
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| complex of byssus mass glands | trating into visceral mass; condensed mass | of foot, not penetrating into visceral mass; condensed mass | | |
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| Special byssus secretions (= byssal hairs) on shell | Absent | Usually conspicuous in small and medium- sized specimens; heavy and adhering, forming commarginal series of thickenings or blunt ser- rations | Weak or absent | Weak | Absent |
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| Posterior adductor | Two unequal portions | Two markedly unequal portions | Two unequal portions | Two unequal portions | Two unequal portions |
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| Posterior byssal retractors | Two main bundles; anterior bundle subdivided further into 2–3 bundles | Two main bundles | Two main bundles | Two main bundles | One main bundle |
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