28o NEW YORK STATE MUSEUM 



O. woolworthanus of the Helderbergian (New Scotland) 

 shaly limestone; it has the long and straight hinge, subsemicircular 

 rarely subelliptical outline, sometimes elongated and in the char- 

 acter of the surface there is comparatively little difference. We 

 observe, however, that inO. woolworthanus the ventral beak 

 is rarely greatly elevated and distorted while this distortion is 

 present in mut. gaspensis, giving the valve at times the 

 aspect ofO. deformis Hall (New Scotland beds). The shell 

 substance is much the thicker in the mutation and the pallial sur- 

 face is vascular. In the dorsal valve the muscle scar of the muta- 

 tion is larger, sharply subdivided and the pallial surface strongly 

 marked with impressions of mantle vessels, the trunks of which 

 are median, departing forward from the front end of the muscle 

 area. In O. woolworthanus the shell is so thin as to seldom 

 show these scars. In both valves the plications are sharply defined 

 about the periphery. On the exterior the mutation shows a rather 

 regular inequality in the striae which in total are probably less in 

 number. The differences are sufficient to indicate a modification 

 of the earliest type expressed inS. woolworthanus. 



The shell attains considerable size, fully that of large examples 

 of O. woolworthanus. 



Lower Devonic. Grande Greve and Shiphead, P. Q, 



Rhipidomella logani nov. 



Prob. Orthis oblata Logan (Billings) Geol. Can. 1863. p. 393 

 Orthis livia Billings (in part). Palaeoz. Foss. 1874. v. 2, pt i, 

 P- 32 



Lens-shaped, subcircular shells, subequally convex in the pos- 

 terior region, the ventral valve depressed anteriorly while the 

 dorsal maintains its convexity. The slope of the surface of the 

 dorsal valve is even in all directions and much more abrupt than in 

 the ventral valve, the former is hence considerably the deeper 

 valve. Little value can be laid upon the characters of the external 

 markings which are in all these species fine, subequal, rounded and 

 sharply elevated lines. 



In the ventral valve the cardinal area is high and rather narrow, 

 the delthyrium broad and the teeth well defined but not conspicu- 

 ously elevated. The adductor scar is broad and flabelliform, 

 extending one half the length of the shell and inclosing narrow 

 and elongate oval diductors. The pallial region is well marked 

 by pallial ridges which inosculate freely. 



