94 MISSISSIPPIAN BRACHIOPODA 



compressed towards the cardinal extremities but the auriculations not 

 sharply differentiated from the body of the valve, in the median portion 

 of the valve the surface is somewhat flattened, but is not depressed in a 

 median sinus; the beak is small and inconspicuous, the umbo smooth 

 at its apex, scarcely extending beyond the cardinal line; cardinal area 

 and delthyrium not observed, the cardinal margin sharply defined, bear- 

 ing two or three spines on each side of the beak. 



Brachial valve moderately or rather deeply concave, the greatest con- 

 cavity posterior to the middle, the surface flattened or only slightly 

 concave towards the cardinal extremities ; the cardinal area very narrow ; 

 the plications do not extend to the beak but leave a small, smooth region 

 at the apex of the valve, in the middle of which, adjacent to the hinge- 

 line, is a small, longitudinal, sharply differentiated, node-like prom- 

 inence. 



Surface of both valves marked by narrow, sharply elevated, rounded 

 costas, which increase by bifurcation on the pedicle valve and by im- 

 plantation on the brachial valve ; they are separated by narrower inter- 

 costal furrows; from three to five costse occupy the space of one milli- 

 meter, about 100 being present at the margin which originate from the 

 bifurcation of about 25 initial ones. Aside from the costae the surface 

 is marked by minute, concentric lines which are more strongly developed 

 across the intercostal furrows than upon the costas themselves, and are 

 often nearly or quite obsolete. 



Remarks. This species resembles C. illinoisensis, but is smaller, with 

 coarser costae which are separated by proportionally narrower and 

 deeper intercostal furrows. The concentric markings also differentiate 

 the species from C. illinoisensis, as well as the smaller number of cardinal 

 spines. The species differs from C. logani, with which it is associated, 

 in its larger size, proportionally less convexity, larger number of costae 

 and less conspicuous concentric markings which are not restricted to 

 the tops of the costas. 



Horizon. Kinderhook. 



CHONETES PLANUMBONUS Meek and Worthen 

 Plate VIII, Figs. 58-62 



1860. Chonetes planumbona Meek and Worthen, Proc. Acad. Nat. Sci. 



Phil., p. 450. 

 1866. Chonetes planumbona Meek and Worthen, Geol. Surv. 111., vol. 2, 



p. 253, pi. 18, figs. la-d. 



Description. Shell concavo-convex, subsemielliptical in outline, the 

 greatest width along the hinge-line, the cardinal extremities nearly rec- 

 tangular, the length from two-thirds to three-fourths the width, the lat- 

 eral margins nearly straight posteriorly, becoming more convex towards 



