BRACHIOPODS. 59 



prominent undefined ridge that sometimes, but not always, 

 imparts a sub-carinate appearance to the central and umbonal 

 regions ; beak small, projecting somewhat beyond that of the 

 other valve ; abruptly pointed and rather distinctly arched, but 

 not strongly incurved; area about twice as high as that of the 

 other valve, and with its sharply defined edges sloping to the 

 lateral extremities of the hinge, directed and arched obliquely 

 backward with the beak ; foramen having nearly the form of 

 an equilateral triangle, but rather narrowed upward to the 

 apex of the beak, and partly occupied by the cardinal process 

 of the other valve. Interior showing the teeth to be moder- 

 ately prominent ; concavity for the muscular impressions very 

 shallow, small, somewhat bifid anteriorly, and not defined by a 

 very distinct marginal ridge ; scars of divaricator muscles 

 apparently narrow, and situated on each side of a shallow 

 mesial depression, which seems to include far back at its pos- 

 terior end those of the very small adductors, merely separated 

 from each other by a hair line ; impressions of ventral adjuster 

 muscles apparently wider and shorter than those of the divari- 

 cators ; stride and the fine granules of the interior as in the 

 other valve. 



Surface of both valves ornamented by numerous distinct 

 radiating striae, that usually bifurcate about three times 

 between the beak and free margins ; posterior lateral striae so 

 strongly curved that a part of them run out on the hinge-line. 

 Numerous very minute, regularly disposed concentric lines 

 may also be seen by the aid of a magnifier, most distinctly 

 defined in the furrows between the much larger radiating 

 striae ; while a few distant, subimbricating, stronger marks of 

 growth are usually seen in adult shells. ( Meek.) 



Horizon and localities Lower Silurian, Hudson shales : 

 Cape Girardeau. 



