436 MISSISSIPPIAN BRACHIOPODA 



length, the hinge-line much shorter than the greatest width of the shell, 

 the cardinal extremities rounded. The dimensions of two nearly perfect 

 specimens are : length of pedicle valve 9.5 mm. and 8.8 mm., length of 

 brachial valve 8.4 mm. and 7.6 mm., width 10.5 mm. and 9.7 mm., thick- 

 ness 5.3 mm. and 6.3 mm., length of hinge-line 4.1 mm. and 4 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 rather abruptly to the cardinal margin and more gently to the lateral and 

 anterior margins, sometimes slightly compressed towards the cardinal ex- 

 tremities ; no mesial sinus present other than the median furrow between 

 the plications of the valve ; the beak rather small, blunt at the extremity, 

 moderately incurved, pierced by a small subcircular foramen; cardinal 

 area small, broadly triangular in outline, gently arched, lying in nearly 

 the plane of the valve, its lateral margins sharply denned ; the delthyrium 

 broadly triangular, occupying nearly one-half the width of the cardinal 

 area along the hinge-line, its apex connecting with the foramen which 

 encroaches entirely upon the beak of the valve, closed by a pair of del- 

 tidial plates ; surface of the valve marked by from six to twelve strong, 

 rounded, simple plications which are separated by deep, rounded grooves 

 about equal in width or a little wider than the plications themselves, the 

 two median plications the strongest, the lateral ones becoming successively 

 smaller towards the cardinal extremities, the outermost ones sometimes 

 becoming almost obsolete. 



Brachial valve more uniformly convex than the pedicle, the greatest 

 depth near the middle, usually slightly compressed towards the cardinal 

 extremities ; the beak incurved and coming in contact with the basal mar- 

 gin of the pseudodeltidium ; plications alternate with those of the opposite 

 valve and entirely similar to them. 



Tn addition to the plications both valves are marked by concentric lines 

 of growth of variable strength and distribution. Shell structure minutely 

 punctate. 



Remarks. This species is rather widespread in its geographical distri- 

 bution in the Mississippi valley, in the faunas of the late Kinderhook, and 

 perhaps passes over into the Burlington limestone. It may be easily 

 recognized by its strong plications, and is not closely similar to any other 

 shell in these faunas. It is rather variable in several particulars, especi- 

 ally in the convexity of the valves and consequent globosity of the shell, 

 and in the number of plications. Because of its variability the species 

 has been described under several names, but a careful examination of as 

 many specimens as possible has led to the conclusion that the differences 

 observed are not constant and are not even of varietal importance, and 

 that all the forms belong to a single species. 



Horizon. Kinderhook. 



