| 21. | The Nautilidae are involute or slightly evolute and generally smooth with straight to sinuous sutures.
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| 22. | Its shell evolute, covered by dense, simple or branching, slightly flexuous ribs.
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| 23. | Its shell is evolute, rapidly expanding, with a depressed, elliptical whorl section.
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| 24. | Shells may be either evolute or involute.
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| 25. | The shell is evolute with a very slightly embracing whorls, ovoid in cross section.
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| 26. | The shell is evolute, early whorls a barrel-shaped cadicone, later become serpenticonic.
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| 27. | "Clymenia " has a closely coiled evolute shell that may be faintly ribbed.
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| 28. | Derivation is from an evolute"
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| 29. | The shell is basically evolute and highly ornamented with close spaced flexious, or sigmoidal, ribbing.
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| 30. | The shell, which grew to be rather large, is evolute, strongly and sharply ribbed.
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