450 DESCRIPTIONS OF NEW SPECIES OF FOSSILS FROM THE 



On casts of this species the palllal line is sometimes so strong as to give a 

 distorted appearance to the specimen. 



This species may be distinguished from Modiomorpha rigida, of this formation, 

 by its greater gibbosity, the less oblique posterior margin, less clearly defined umbonal 

 ridge, and the more prominent beaks. 



Formation and locality. Chemung group, Tioga village, Tioga county, Penn- 

 sylvania. 



MODIOLOPSIS SUBRHOMBOIDEA Simpson, n. sp., Fig. 17. 



Shell of medium size, rhomboid ovate in outline ; length twice the height ; basal 

 margin slightly convex along the middle, curving to the extremities ; posterior mar- 

 la.. ^- gin abruptly rounded below, somewhat more gradually recurving 



\--^''-" :^ to the cardinal line ; cardinal margin slightly arcuate ; anterior 



Kj -^^^TtlS^ . ii/ margin sharply rounded. 

 /'." ""^ Tri£89 Valves flattened, greatest convexity at the umbonal ridge. 



Hinge line slightly oblique, extending a little more than two-thirds the length of 

 the shell. 



Beaks appressed, situated about one-fourth the length of the shell from the 

 anterior end ; umbonal ridge not distinctly defined ; posterior slope rounded, becom- 

 ing flattened just before reaching the cardinal line. 



Surface marked by fine concentric linee, and at irregular distances apart by 

 varices of growth. 



The anterior muscular impression is moderately large, well marked, and situated 

 just within the anterior margin below the beak. The best preserved specimen has a 

 length of 24 mm., and a height of 13 mm. 



This species may be distinguished from M. suhcarlnatus by the less clearly 

 defined umbonal ridge, the somewhat arcuate hinge line, and the absence of a constric- 

 tion in the basal margin. 



Formation and locality. Clinton shale, above fossil ore, at McKee's ore bank, 

 north-east of McKee's house, Ferguson valley, seven miles north-west of Lewistown, 

 Milllin county, Pennsylvania. 



GONIOPIIOllA CURVATA Simpson, n. sp.. Fig. IS. 



Shell trapezoidal, medium size or smaller ; length usually about twice the height, 

 sometimes a little less; anterior end i-apidly declining from the beaks, abruptly 

 rounded below; basal margin gently rounding, sinuate a little anterior to the middle; 



