IIo HISTORICAL PALAEONTOLOGY. 
and fig. 51, ¢ and @), are usually more or less transversely- 
oblong or subquadrate, the two valves (as more or less in all 
Fig. 51.—Lower Silurian Brachiopods. a, Strophomena alternata, Cincinnati Group, 
America ; 6, Strophomena filitexta, Trenton and Cincinnati Groups, America; c, Orthis 
zestudinaria, Caradoc, Europe, and America; d@, a’, Orthis plicatella, Cincinnati 
Group, America; e, ¢’, e’, Leptena sericea, Llandeilo and Caradoc, Europe and Ame- 
rica. (After Meek, Hall, and the Author.) 
the Brachiopods) of unequal sizes, generally more or less con- 
vex, 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 attach- 
ing the shell to some foreign object. The genus St/ophomena 
(fig. 50, d, and 51, a and 2) is very like Ortfzs in general char- 
acter ; but the shell 1s 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 Leffena, again (fig. 51, e), the shell 
is like Strophomena in many respects, but generally compara- 
tively 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 Lingua, Crantia, 
Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella, and 
Athyris ; but none of these can claim the importance to which 
the three previously-mentioned groups are entitled. 
The remaining Lower Silurian groups of JZo/lusca can be 
but briefly glanced at here. The Bivalves (Zamellibranchiata) 
find numerous representatives, belonging to such genera as 
