THE SHELL-LIMESTONE OP DURHAM. 



Ul 



characters, as also its position, which is not constant, being 

 sometimes nearer one extremity than the other. 



Bairdia plebeia, Reuss. PI. IX., figs. 1 and 2, woodcuts la, 

 16, Ic. 



^. ^/e5m, Eeuss. " Jahreshericht der Wetterau. 

 Gesell. 1854," p. 67. Kirkhy, " Ann. Nat. His.," 

 3rd ser., vol. ii., p. 324, pi. X., figs. 1 and 2. 

 Length, 1-1 3th of an inch ; height, l-30th of an inch. 

 Carapace subdeltoid, rather inflated centrally. Surface 

 smooth. Dorsal margin prominently convex ; postero- 

 dorsal region sloping abruptly, slightly concave ; antero- 

 dorsal region sloping more gradually, straight, or slightly 

 concave. Ventral margin straight or slightly incurved, 

 more or less convex towards each extremity. Anterior 

 extremity obliquely rounded, and rather produced. Pos- 

 terior extremity acute, rostrated. Lateral contour regularly 

 lenticular ; greatest diameter (of lateral contour) two-thirds 

 of the height. Flange of ventral margin sub -central. 

 Dorsal overlap strong. 

 B. pleheia enjoys a great latitude of form. Figs. 1 and 2 

 represent what are considered to be type-sjDecimens. Several of 

 the most important varieties will be separately noticed, and their 

 differences pointed out. The modification of certain specific cha- 

 racters in the most aberrant varieties induced me at first to de- 

 scribe them as distinct species. But on a careful study of a very 

 full series of specimens, I have reduced them to their more ap- 

 propriate position of varieties. In the correction of these false 



