[Illustration: Fig. 51.--Lower Silurian Brachiopods, a, _Strophomena
alternata_, Cincinnati Group, America; b, _Strophomena 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.
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