The Lamp-shells or Brachiopods are so numerous, and present such varied types, both in this and the succeeding period of the Upper Silurian, that the name of “Age of Brachiopods” has with justice been applied to the Silurian period as a whole.  It would be impossible here to enter into details as to the many different forms of Brachiopods which present themselves in the Lower Silurian deposits; but we may select the three genera Orthis, Strophomena, and Leptoena for illustration, as being specially characteristic of this period, though not exclusively confined to it.  The numerous shells which belong to the extensive and cosmopolitan genus Orthis (fig. 50, a, b, c, and fig. 51, c and d), are usually more or less transversely-oblong or subquadrate, the two valves (as more or less in all the Brachiopods) of unequal sizes, generally more or less convex, and marked with radiating ribs or lines.  The valves of the shell are united to one another by teeth and sockets, and there is a straight hinge-line.  The beaks are also separated by a distinct space ("hinge-area"), formed in part by each valve, which is perforated by a triangular opening, through which, in the living condition, passed a muscular cord attaching the shell to some foreign object.  The genus Strophomena (fig. 50, d, and 51, a and b) is very like Orthis in general character; but the shell is usually much flatter, one or other valve often being concave, the hinge-line is longer, and the aperture for the emission of the stalk of attachment is partially closed by a calcareous plate.  In Leptoena, again (fig. 51, e), the shell is like Strophomena in many respects, but generally comparatively longer, often completely semicircular, and having one valve convex and the other valve concave.  Amongst other genera of Brachiopods which are largely represented in the Lower Silurian rocks may be mentioned Lingula, Crania, Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella, and Athyris; but none of these can claim the importance to which the three previously-mentioned groups are entitled.

[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.