ophomena filitexta,
Trenton and Cincinnati Groups, America; c, _Orthis testudinaria_,
Caradoc, Europe, and America; d, d', _Orthis plicateila_, Cincinnati
Group, America; e, e', e'', _Leptoena sericea_, Llandeilo and
Caradoc, Europe and America. (After Meek, Hall, and the Author.)]

The remaining Lower Silurian groups of _Mollusca_ can be but
briefly glanced at here. The Bivalves (_Lamellibranchiata_) find
numerous representatives, belonging to such genera as _Modiolopsis,
Ctenodonta, Orthonota, Paloearca, Lyrodesma, Ambonychia_, and
_Cleidophorus_. The Univalves (_Gasteropoda_) are also very numerous,
the two most important genera being _Murchisonia_ (fig. 52) and
_Pleurotomaria_. In both these groups the outer lip of the shell
is notched; but the shell in the former is elongated and turreted,
whilst in the latter it is depressed. The curious oceanic Univalves
known as the _Heteropods_ are also very abundant, the principal
forms belonging to _Bellerophon_ and _Maclurea_. In the former
(fig. 53) there is a symmetrical convoluted shell, like that of
the Pearly Nautilus in shape, but without any internal partitions,
and having the aperture often expanded and notched behind. The
species of _Maclurea_ (fig. 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
wat