80 [ Senate 



ORTHIS LEUCOSIA ( n. s.). 



Shell broad ovate, greatest breadth below the middle, obtusely pointed at the 

 beaks. Valves moderately convex : cardinal area short and small; beaks appro- 

 ximate, pointed and incurved. Ventral valve depressed convex, most gibbous at 

 the umbo and flattened towards the front, without a distinct mesial sinus, but 

 sometimes having a broad shallow curve at the anterior margin of the shell : 

 foramen broad triangular. Dorsal valve much the more gibbous, the greatest 

 convexity above the middle, marked along the middle by a very slight depression 

 which is sometimes obsolete : area smaller than that of the opposite valve, curved 

 and slightly overhanging. Surface marked by fine radiating bifurcating striae, 

 crossed by strong distant lamellose lines of growth, and interrupted by the 

 openings of the pores. Interior of ventral valve marked by a strong, foliate, 

 somewhat elongate muscular impression. 

 This species is allied to O. vanuxemi, but differs in the cardinal margin being 



nearly straight fi-om the beaks to nearly one-third the length of the shell, while in 



that species it is usually regularly curved : it differs also in the area being much 



smaller, and the beaks incurved and closely approximate. 



Geological formation and locality. Shales of the Hamilton group : Ontario 



county, N.York. 



ORTHIS SOLITARIA(n.s.). 



Shell small, subcircular or broadly ovate : valves unequally convex; hinge-line 

 somewhat more than one-half the greatest width of the shell. Ventral valve 

 highly convex : beak small, prominent, slightly incurved ; area low and well 

 defined. Dorsal valve depressed convex, most prominent near the umbo; a broad 

 shallow mesial sinus at the anterior margin, which does not extend beyond the 

 middle of the shell : beak small, not prominent; area linear. Surface marked by 

 fine radiating bifurcating striae and strong concentric lines of growth. 

 This shell is of the type of O. elegantula. It differs from any other in the Ha- 

 milton group, except 0. lepidus, in having the ventral valve more convex than the 

 dorsal, the sinus being on the dorsal valve. From O. lepidus it differs in being 

 longer than wide, with less prominent beak and smaller area : it is also a larger 

 species, being more than half an inch in diameter. 



Geological formation and locality. Shales of the Hamilton group : Livingston 

 county, N.York. 



ORTHISINA ARCTOSTRIATA ( n.s.). 



Shell small, semicircular or semielliptical : hinge-line straight, nearly equal to 

 the greatest width of the shell. Ventral valve irregularly gibbous : beak small, 

 pointed, and truncate from its adhesion to foreign substances; area moderate, 

 slightly arcuate, and somewhat irregular on the two sides; pseudo-deltidium 

 broadly triangular, c' osed. Dorsal valve depressed convex; area narrow linear. 

 Surface marked by 5 trong sharp close radiating crenulated striae, which increase 

 by interstitial additicL , and crossed by strong concentric lines of growth. 

 Geological formatior ...'.d locality. Shales of the Hamilton group : Ontario 



county, N.York. 



