FOSSILS OF THE HUDSOlSr RIVEE GROUP. 93 



almost concave between the two points, and giving a rather abrupt or 

 sub-angular umbonal ridge, which curves in its course from the beaks to 

 the postero-basal angle. The middle of the shell is marked by a broad, 

 usually undefined, but sometimes distinct, oblique, mesial sulcus, extend- 

 ing from the beaks to the basal margin, and occupying almost the entire 

 space between the umbonal ridge and the antefior prominence. Cardinal 

 slope broad, marked by a faint secondary ridge extending from behind 

 the beaks to the middle of the postero-cardinal body. Surface of the shell 

 marked by fine, somewhat even, concentric lines of growth, but without 

 evidence of concentric ridges or plicse. 



This species differs from C Miamiends in the much less ventricose 

 valves, less prominent beaks, and umbonal region, in the sub-parallel car- 

 dinal and basal margins, and in the absence of concentric ridges. From 

 Sedgwickia (Grammysia) neglecta, Meek, it differs in the less prominent 

 beaks, in the straight or broadly sinuate basal margin, in the less con- 

 vexity of the central region of the valves, and in the absence of the con- 

 centric undulations of the surface. 



Formalion and locality : In the soft shales of the Hudson River group, near Waynes- 

 ville, Ohio. 



Genus ORTHODESMA (new gen.). 



More or less elongate, bivalve shells, having the hinge line straight 

 and generally extended posterior to the beaks, but contracted or bent 

 beneath or anterior to them ; hinge plate apparently edentulous ; valves 

 united by an external ligament extending to a greater or less distance 

 along the posterior cardinal margin. Posterior muscular scar elongate 

 ovate, anterior scar smaller, both faintly marked; pallial line simple. 

 Shells thin, marked externally with irregular concentric plicse. Type, 

 Orthodesma recta. 



The shells for which the above generic name is proposed are thin and 

 generally compressed. They have usually been referred to the genus 

 Orthonota, Conr., or to Modiolopsis, Hall, but do not possess the generic 

 features of either. From Orthonota they differ in having the hinge line 

 bent or contracted beneath or just anterior to the beaks, instead of being 

 continued in a straight line to the anterior extremity of the shell, as in 

 Orthonota undulata, the species upon which Mr. Conrad founded the 

 genus. From Modiolopsis they differ in the absence of hinge teeth, so far 



