342 ' Report of the State Geologist. 



anterior edge. The shells are usually flattened, but where the 

 form is retained the pedicle-valve shows a hinge-line whose leng^th 

 is somewhat less than one half the transverse 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 inconspicuous. 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 anasto- 

 mosing 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 margias the surface slopes 

 rather abruptly and without much curvature. On the interior 

 the cardinal process and crural plates are not prominently elevated; 

 the niuscular 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 stride, from 110 to 130 in number, which are 

 crenulated by minute concentric lines and crossed at intervals by 

 coarser lines of growth. The surface was originally covered 

 with short spinules, which are rarely preserved. This shell has 

 heretofore been commonly referred to Orthis Michelini, Leveille. 



Keokuk group (Knobstone formation). Button-mould Knohs, 

 Kentucky. 



Orthis (Schizophoria) senecta. 



Plate 4, figs. 23, 23. 



Orthis (Schizophoria) senecta, Hall. Palaeontology of N. Y., vol. VIII, pt. l,p 

 343, pi. vi a, figs. 23, 24. 1892. 



Shell subquadrate or transversely elliptical, resupinate, un- 

 equally biconvex. The pedicle-valve is depressed convex in the 

 umbonal 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 stride. 



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



Clinton group. Reynale^s Basin, Niagara county, New Yorh. 



