26 



PAL/EONTOLOGY 



its deltidium and hinge-teeth are rudimentary. It shows, 

 however, the long straight hinge-line and wide shell that 

 characterize so many of the Palaeozoic Articulata. 



In the Ordovician, and still more in the Silurian, this 

 sub-order attains very great importance, and some of its 

 most familiar members deserve detailed treatment. 



Leptcena rhomboidalis (Fig. 6) of the Wenlock Limestone 



V A 



FIG. 6. LEPT^NA RHOMBOIDALIS, WILCKENS. 

 (Natural size.) 



a, Dorsal view ; b, interior of ventral valve ; c, posterior view ; d, interior 

 of dorsal valve ; Add., adductor impression ; Ch., chilidium (showing 

 median pedicle-groove); C.P. , cardinal process; D, dehhyriuni ; 

 D.A., cardinal area of dorsal valve; Div. , divaricator impression; 

 V.A., cardinal area of ventral valve; V. /., umbo of ventral valve. 

 (Original.) 



is a broad shell (length : breadth = about 2:3) with a semi- 

 elliptical outline, the long straight hinge-line forming the 

 greatest breadth of the shell. The early (posterior) part 

 of the shell is flat, but after growing to a length of about 

 two centimetres a sudden change takes place and the 



