THE BRACHIOPODA 29 



one another. The failure to discriminate between 

 homceomorphs has frequently led to mistakes in the 

 correlation of strata. 



Productus productus (commonly, but wrongly, termed 

 Productus martini) is one of many species of Pvoductus that 

 abound in the Lower Carboniferous Limestones and 

 Shales. In general form it is not unlike Leptcena, but the 

 convex ventral valve shows a stronger and more uniform 

 curvature, and its umbonal region is much larger and 

 more rounded ; there is no cardinal area and no delthy- 

 rium. The dorsal valve is for the first two centimetres 

 from the umbo, almost flat, very slightly concave ; it also 

 has no cardinal area and its umbo does not project above 

 the hinge-line. The hinge-line is long and straight, but 

 less than the greatest breadth of the shell. The profile 

 of the ventral valve first forms an arc which is almost a 

 semicircle, of which the dorsal valve forms the chord, and 

 at two centimetres from the umbo the valves meet as 

 though the shell were complete. From this distance on, 

 however, the ventral valve is continued with very slight 

 curvature for a considerable length, while the dorsal valve 

 bends abruptly and continues in close contact with the 

 ventral, the two valves thus forming a sort of flange with 

 no space between them when the shell is closed. This 

 flange is very easily broken off during extraction of the 

 fossil from the rock, and what remains has all the 

 appearance of a perfect shell, two centimetres long and 

 about 2*5 centimetres broad. 



The surface is ornamented with close-set radiating 

 ribs, rounded in section, with a few concentric corruga- 



