64G Systematic Paleontology — Upper Devonian 



This species is distinguished from allied Devonian forms of the genus 

 by its low umbonal ridge and direction of its posterior margin. It is a 

 well-known Chemung and Ithaca fossil in New York. 



Leng-th 25 mm. ; height 20 mm. 



Occurrence. — Jennings Formation, Chemung Member. Xear Deer 

 Park Station. 



Collection. — Maryland Geological Survey. 



SCHIZODUS oherni n. sp. 

 Plate LXIV, Figs. 1-14 



Description. — Shell small, varying from rhomboidal to suborbicular, 

 the latter shape more frequent in young shells ; length nearly equal or 

 slightly greater than width; hinge-line short, convex. Posterior margin 

 gently curved, descending obliquely to postinferior extremity, or rounded; 

 anterior margin curving to ventral margin ; postinferior extremity ex- 

 tended in some individuals, or in other cases but slightly so. Valves 

 convex to gibbous; beaks anterior or nearly subcentral in position, gibbous, 

 elevated above hinge-line, incurved; umbonal slope rounded to angular; 

 surface descending abruptly to postcardinal margin, this portion being 

 convex to concave; convex towards anterior and ventral margins. 



Surface smooth, bearing faint concentric striae. Test thick, especially 

 so near umbo. Interior shows deep anterior and posterior muscular scars, 

 the anterior scar a little higher; right valve bearing two diverging teeth 

 with smooth edges, a slight depression between them ; left valve with two 

 corresponding depressions separated by a slightly bifid elevation which 

 appears as a bifid pit beneath beak in casts of interior. 



Length 15-17 mm. ; height 14-16 mm. in large individuals. 



This species is characterized by its strong muscular scars, small size, 

 small umbo neatly incurved over hinge, and oblique shape. The test is 

 quite thick over umbo so that the exterior appears much more gibbous 

 than casts of interior. The shape is quite variable. It has the character- 

 istic hinge of the genus tichizodus and is not clearly referable to any pre- 

 viously described species. Specimens occurring at higher horizons are 

 usually somewhat larger than those from the lower horizons. 



This species occurs chiefly in conglomeratic sandstones. 



