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. 



J^HELL longitudinal, more or less gibbose, sometimes 

 neai'ly equalvalved ; but sometimes with very unequally 

 elongated beaks ; the anterior side is more or less pro- 

 duced, and supports the strraght 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 laminae 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 recognized ; but \a 

 consequence of that paper not being printed until 1823, 

 much uncertainty has existed^ and several misrepresen- 



