57 
INOCERAMUS. 
Gen. Char. A free, more or less inequilateral, 
irregular, bivalve ; hinge a marginal, sub- 
cylindrical, transversely sulcated callus, 
supporting a ligament ; beaks conspicuous, 
at one end of the hinge. 
Shell longitudinal, more or less gibbose, sometimes 
nearly equalvalved ; but sometimes with very unequally 
elongated beaks ; the anterior side is more or less pro- 
duced, and supports the straight hinge callus upon its 
edge ; the posterior side is sometimes lobed, at other 
times flat or convex ; the hinge consists of a more or 
less cylindrical callus, produced by a rapid and very 
considerable increase in the thickness of the shell at its 
edge ; it is concave on one side, and transversely sul- 
cated to receive the ligament ; it is the same in both 
valves ; there is no opening for the passage of a byssus ; 
the shell consists of closely pressed laminae, composed of 
perpendicular fibres ; the edges of these laminge form 
concentric lines upon the surface, but are not prominent ; 
some species have a lining of pearl. In all, the shell is 
very thin at and around the beaks, but becomes very 
thick at the edges, near the hinge, and towards the front. 
This Genus may be divided into two sections, the first 
containing the species with short beaks, and nearly equal 
valves ; the second those with elongated beaks and un- 
equal valves. 
Nine years have elapsed since my lamented Father, in 
a paper read before the Linnean Society in 1814, made 
known the characters by which this Genus formerly re- 
ferred to Patella, and afterwards, in consequence of its 
fibrous stucture, to Pinna, might be recognised ; but in 
consequence of that paper not being printed until 1823, 
much uncertainty has existed, and several misrepresent 
