54) are found both in North America
and in Scotland, and are exclusively confined to the Lower Silurian
period, so far as known. They have the shell coiled into a flat
spiral, the mouth being furnished with a very curious, thick,
and solid lid or "operculum." The Lower Silurian _Pteropods_,
or "Winged snails," are numerous, and belong principally to the
genera _Theca, Conularia_, and _Tentaculites_, the last-mentioned
of these often being extremely abundant in certain strata.
[Illustration: Fig. 52.--_Murchisonia gracilis_, Trenton Limestone,
America. (After Billings.)]
[Illustration: Fig. 53.--Different views of _Bellerophon Argo_,
Trenton Limestone, Canada. (After Billings.)]
[Illustration: Fig. 54.--Different views of _Maclurea crenulata_,
Quebec Group, Newfoundland. (After Billings.)]
[Illustration: Fig. 55.--Fragment of _Orthoceras crebriseptum_,
Cincinnati Group, North America, of the natural size. The lower
figure section showing the air-chambers, and the form and position
of the siphuncle. (After Billings.)]
[Illustration: Fig. 56.--[14] Restoration of Orthoceras, the shell
being supposed to be divided vertically, and only its upper part
being shown. a, Arms; f, Muscular tube ("funnel") by which
water is expelled from the mantle-chamber; c, Air-chambers;
s, Siphuncle.]
[Footnote 14: This illustration is taken from a rough sketch
made by the author many years ago, but he is unable to say from
what original source it was copied.
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