110 [Senate 



Orthis propinqtja. 



Pal. N.Y. Vol. iv. 



Shell transversely oval, rather gibbous; hinge straight, equalling 

 about one half the greatest breadth of the shell : dorsal valve 

 the larger, gibbous especially in the umbonial region ; beak 

 equalling or sometimes extending a little beyond that of the 

 opposite valve, incurved ; dorsal area incurved : ventral valve 

 depressed -convex, somewhat prominent towards the beak, 

 having a broad faintly defined depression near the anterior 

 margin, giving it a straight or somewhat emarginate outline ; 

 foramen narrow ; area broad, triangular, arcuate. Surface marked 

 by numerous fine irregular striae, apparently increasing chiefly 

 by interstitial additions, and crossed by fine indistinct concen- 

 tric lines and a few stronger w^rinkles of growth. 



This species approaches so closely O. multistriata of the Pentamerus 

 limestone, that it is very difficult to distinguish them, the form and surface 

 characters being generally almost precisely the same. Sometimes, however, 

 the beak and area of the ventral valve of this species is a little more ar- 

 cuate than in 0. multistriata, and that of the dorsal valve is usually more 

 gibbous, while internally they present some well-marked differences. In this 

 shell, the vascular impressions bifurcate once, twice, or even three times, 

 before reaching the border ; while those of O. multistriata appear to pass 

 down the front without bifurcation. 



Orthis tulliensis, of the TuUy limestone, is also another form very 

 difficult to distinguish from this : it is, however, generally more gibbous, 

 and presents internal differences ; the divisions of the vascular impressions 

 pass down the front of the dorsal valve nearly parallel to each other, or 

 slightly converging ; while those of the species under consideration, as well 

 as of O. viultistriata, diverge distinctly. 



Geological 'position and locality. Limestone of the Upper Helderberg 

 group. New- York and Ohio. 



