13B 



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 shghtly 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 &st 

 sight to be smooth. On account of the distoi-ted 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. 



Obthis Iphigenia. (N. sp.) 



a b 



Fig. HO. 

 Fig. 110. — Orthis Iphigenia. — a, Ventral view ; b, aide view. 



Description. — Transversely sub-elhptical ; hinge-line one-third shorter 

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

 irregularly or miiformly 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 1\ to 3 hues from the front margin ; area small, about 

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



