ORTHIDES OP THE UPPER HELDERBERG ROCKS. 43 



convex in the upper part of the centre, depressed in the middle, and 

 concave towards the base and lower lateral margins? cardinal angles 

 deflected : ventral area inclined backwards, of medium height ; fora- 

 men wider than high ; ventral beak projecting a little beyond that of 

 the opposite valve. 

 Surface marked by subequal rounded radiating striae, which bifurcate 

 two or three times before reaching the margin. There are about ten 

 striae in the space of two lines near the beak, eight in the middle of 

 the shell, and five or six near the margin. Length of specimen a little 

 more than an inch and a half; greatest width about an inch and three- 

 fourths. 



This species is extremely rare, and I have but one good example. In this the 

 depression of the ventral valve below the middle may be in part due to accident, 

 and I have indicated this by an interrogation in the description. It is readily 

 distinguished from any other species in these strata, by the long straight hinge- 

 line and subequal valves. 



In form and general aspect it resembles a Streptorhynchus, to which genus it 

 may possibly belong, though the striae have the character of those of an Orthis. 



Geological formation and locality. In the Corniferous limestone at Williamsville, 

 N.York. 



C. Species of the type of Orthis resupinata, O. multistriata, etc., with the dorsal valve 



extremely convex. 



Orthis propinqua. 



PLATE V. 

 Orthis propinqua : Hall, Teath Report on the State Cabinet, 1857, p. 110. 



Shell somewhat transversely elliptical or subquadrate, the front truncate 

 or emarginate, and the sides little curved, varying from moderately to 

 extremely gibbous : hinge-line equalling or a little greater than half 

 the width of the shell. Dorsal valve the larger and much the more 

 gibbous ; the greatest convexity a little above the middle of the length, 

 curving abruptly to the sides and beak, and a little more gently to the 

 front ; sometimes a little concave just within the cardinal angles : 



