133 



elliptical; hinge-line equal to the greatest width of the shell; cardinal 

 angles rectangular or nearly so; sides straight or gently convex, and 

 parallel in the upper half; all the front half of the shell broadly rounded. 

 Width 10 lines; length 8 lines. 



The shell is very thin and fragile. The surface is covered with minute 

 closely-crowded radiating striae, barely visible to the naked eye. Under 

 the glass there appear to be about 20 of these in the width of one line. 

 They are slightly undulated ; the new ones come in by implantation, and 

 the whole are crossed by extremely minute concentric striae. The striae 

 are of a uniform size, and so small that to the eye the shell appears at first 

 sight to be smooth. On account of the distorted condition of the specimens, 

 I cannot determine which valve is the convex one, but it appears to be the 

 ventral. 



Locality and Formation Observation Cape, Anticosti. Hudson River 

 group. 



Collector. J. Richardson. 



ORTHIS IPHIGENIA. (N. sp.) 



j. no. 



Fig. 110. Orthis Iphigenia. a, Ventral view ; 6, side view. 



Description. Transversely sub-elliptical ; hinge-line one-third shorter 

 than the greatest width of the shell ; cardinal angles rounded ; sides 

 irregularly or uniformly rounded ; greatest width about the middle or a 

 little in front thereof; front margin broadly rounded or very gently 

 convex. Ventral valve depressed convex, greatest elevation on the umbo 

 just in front of the beak, descending with a flat or gently concave slope to 

 the cardinal angles ; the central region either gently convex, flat, or gently 

 concave ; a wide concave shallow mesial sinus, which usually terminates 

 abruptly at from 1J to 3 lines from the front margin ; area small, about 

 half the width of the shell, slightly concave, and a little overhanging the 

 hinge-line ; umbo with its most elevated point distant from the beak a 

 little less than the height of the area ; beak small, depressed about one-third 

 below the most elevated point of the shell, scarcely distinct from the 



