1 8 DAY, Variation in a Carboniferous Brachiopod. 



DESCRIPTION OF PLATE I. 



( All photographs arc \ths nat. size except L.J 



A and B. —Pair of shells representing the extremes of variation 

 in breadth, in shells of equal length. 



C and D. — Pair of shells representing the extremes of variation 

 in depth, in individuals of equal breadth. 



E and F. — Pair of shells representing the extremes of variation 

 in depth, in shells of equal length. 



G, ff, and /. — Shells illustrating the twisting of the ventral 

 beak. N.B. — Each shell was placed with the 

 hinge line strictly horizontal. 



/ and X — Pair of shells illustrating the extremes of variation 

 in the relative size of the ventral beak. 



L. — Microphotograph of the surface ornament. x 6*3 diams. 



