58 MISSISSIPPIAN BBACHIOPODA 



12 mm., length of brachial valve 9.6 mm., greatest width 12.4 mm., length 

 of hinge-line 10.8 mm., height of cardinal area, 2.8 mm., thickness 5.6 mm. 



Pedicle valve somewhat distorted, moderately convex, most prominent 

 posteriorly, the surface a little compressed towards the cardinal ex- 

 tremities, with no mesial sinus ; the beak pointed, extending conspicuously 

 beyond the cardinal margin posteriorly ; cardinal area of moderate height, 

 flat, inclined posteriorly, the lateral margins sharply defined and angular, 

 sloping in nearly straight, slightly concave or slightly convex lines from 

 the beak to the cardinal extremities; the delthyrium broadly triangular, 

 closed by a convex deltidium. Internally the hinge-teeth are supported 

 by short dental lamellae, no median septum present. 



Brachial valve rather strongly convex, the greatest convexity near the 

 middle, compressed towards the cardinal extremities ; the umbonal region 

 prominent, but not protuberant posteriorly beyond the cardinal margin, 

 the surface curvature more gentle towards the lateral and anterior 

 margins. 



The surface markings of both valves consist of a series of rather strong 

 radiating costse from one to two millimeters apart, which originate in 

 the umbonal region and which are absent towards the cardinal extrem- 

 ities, between the coarser costse and extending to the cardinal "extremities, 

 the surface is covered by much finer radiating costaa. The surface also 

 marked by regular, concentric lines of growth of moderate strength. 



Remarks. This species was originally described as a member of the 

 genus Derbya, with a query, by Hall and Clarke. The holotype of the 

 species shows neither the presence or absence of a median septum in the 

 pedicle valve, but another specimen, a pedicle valve with a perfectly clean 

 interior, from the same locality as the holotype, shows conclusively that 

 no such septum is present. The dental lamellae in the same specimen are 

 better developed than in the typical Schuchertella where they are reduced 

 to mere thickenings of the inner surface of the valve along the margins 

 of the delthyrium, being distinct plates, although very short. In this 

 character, then, the species resembles the members of the genus 

 Schellwienella, but it does not possess the resupinate form of typical 

 members of that genus. The species is, therefore, intermediate in its 

 characters between these two genera, and is placed in Schuchertella some- 

 what arbitrarily. 



The species is especially characterized by the strong radiating costae 

 which occur at intervals, with the much finer ones between. Occasionally 

 specimens of the truly septate Orthotetes kaskaskiensis exhibit this same 

 characteristic, though not to such a conspicuous degree, but that species 

 is always a much larger shell in its adult condition, than the largest ex- 

 amples of 8. costatula which have been observed. 



Horizon. Chester group. 



