That margin in the scuta and terga which opens and _shuts_ for the exsertion and retraction of the cirri, I have called the Occludent margin. [Please select]
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Scuta[3] furnished only with an adductor muscle: other valves, when present, not united into an immovable ring. [Please select]
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In several genera, namely, Pæcilasma, Dichelaspis, Conchoderma, and Alepas, the scuta show a tendency to be bilobed or trilobed. [Please select]
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For this same end, the shape of the scuta and carina, but not of the other valves, comes into play. [Please select]
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Scuta and terga with one or more diagonal lines of dark greenish-brown, square, slightly depressed marks. [Please select]
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Surfaces smooth, with traces of very fine lines radiating from the umbones, sometimes rather plain on the basal part of the scuta. [Please select]
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There are sometimes two or even three rows on the scuta. [Please select]
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The latter, in well preserved old specimens, may often be detected on the umbones of the scuta, terga, and carina, but not on the umbones of any other valves. [Please select]
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The adductor muscle is always attached to a point not far from the middle of the scuta, and it generally has a pit for its attachment. [Please select]
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