558 Contrihutiotis to Invertebrate Palaeontology. 



Genus NYASSA H. & W. 



Preliminary Notice of Lamellibranchiata shells of the Upper Helderberg Hamil- 

 ton and Chemung Groups, etc., Albany, N. Y. 18(J9, p. 28 Cgeueric de- 

 scription omitted) ;— Whitf., Ann. N. Y. Acad. Sci., 1882, p. 21G. 



Shells bivalve, very oblique and transversely ovate in form. Posterior 

 hinge-plate narrow, bearing from one to four long, slender, ridge-like teeth. 

 Anterior plate broad, marked by numerous small point-like teetli with inter- 

 mediate dejjressions, arranged somewhat radiating from the middle of its inner 

 border. Adductor muscles two, one at each extremity. Pallial line entire. 

 Ligament internal. Type, N. argutu. Name, mytliological. Geological range, 

 so far as known, Devonian. Family relations apparently near Mecjalomus Hall, 

 and Megalodon Sowerby. ^ 



IVyassa arguta. 



Plate XI, fig. 18. 



Nyassa art/ida 11. and W. Prelim. Notice of the Lamellib. Shells of the Upper 

 Held., Hamilton and Chemung Groups, etc., Albany, N. Y., Dec. 1869, 

 p. 28 ;— Whitf., Ann. N. Y. Acad. Sci., 1882, p. 216. 



Shell of medium size, transversely sub-ovate or sub-trapezoidal, much longer 

 than high. Valves moderately ventricose, most prominent along the umbonal 

 ridge, which is rather strongly arcuate and sub-angular. Beaks rather small 

 and appressed, slightly incurved, and situated near tlie anterior end. Surface 

 of the valve generally declining from the umbonal ridge to the basal line, and 

 with a slight sinus or sulcus below the ridge, which gradually widens toward 

 the margin of the shell, where it causes a broad, but not marked, emargina- 

 tion in the border of the shell. Cardinal slope narrow and abrupt ; hinge- 

 line arcuate ; posterior end of the shell narrowed ; anterior end broad, rounded, 

 and slightly excavated below the beaks. 



Surface of the shell marked by concentric lines of growth parallel to the 

 margin of the valve, and often forming rather strong, irregular varices, most 

 distinctly marked on the anterior half of the shell. 



The Ohio specimens, although preserved in an entirely different 

 matrix, are yet such exact counterparts of the New York shells 

 that no question can exist of their positive identity. 



Formation and Locality. — In limestone above the "Bone-bed" 

 in Tully Township, Marion Co., Ohio. The specimen figured is 

 from the State Cabinet at the State University, Columbus, Ohio. 



