BRACHIOPODA. 343 



diameter of the shell. The beak is acute, the umbo full but not conspicuous. 

 Along the center of the valve is a broad, low sinus, frequently very in- 

 conspicuous. The interior of this valve is characterized by the relatively 

 small area covered by the muscular scars, a feature in which it resembles 

 0. Peloris of the Schoharie grit. The pallial region is pitted or covered with 

 faint, closely anastomosing ridges. On the brachial valve the median sinus 

 begins at the apex and becomes very pronounced as it widens anteriorly. 

 From the ridges forming its lateral margins the surface slopes rather abruptly 

 and without much curvature. On the interior the cardinal process and 

 crural plates are not prominently elevated ; the muscular area is small, 

 quadripartite, the lateral pairs of scars being separated by a broad, thick 

 ridge. 



Surface of both valves covered by a great number of fine radiating, hollow 

 striae, from 110 to 130 in number, which are crenulated by minute con- 

 centric lines and crossed at intervals by coarser lines of growth. The 

 surface was originally covered with short spinules, which are rarely pre- 

 served. This shell has heretofore been commonly referred to Orthis 

 Michelini, Leveille. 



Keokuk group (Knobstone formation). Button-mould Knobs, Kentucky. 

 Orthis (Schizophoria) senecta, sp. nov. 



PLATE VIA, FIGS. 23, 24. 



Shell subquadrate or transversely elliptical, resupinate, unequally biconvex. 

 The pedicle-valve is depressed convex in the urabonal region and develops 

 a broad, low median sinus toward the anterior margin. The brachial valve is 

 the more convex and slopes evenly toward the lateral margins, the median 

 region being rendered slightly more prominent by an obscure fold. 



Internal markings as in other members of Schizophoria. 



External surface covered with fine, subequal, closely covered radiating striae. 



Length of a typical example, 17 mm.; width, 21 mm. 



Clinton group. Reynale's Basin, Niagara county. New York. 



