CRETACEOUS PERIOD. 183 



tinct radiating striae, and occasionally still more indistinct traces of similar 

 ones below that angle, all of which are more discernible upon the anterior 

 than upon the posterior part of the shell. More or less of the test is pre- 

 served upon all the specimens ; but no trace of the pearly layer has been 

 detected, all the test having the usual prismatic structure. 



Some of the largest examples measure seven and a half centimeters in 

 width along the posterior margin, and they must have been not less than 

 seventeen centimeters in length when entire. The large size, proportionally 

 great width, and angular aspect of this shell distinguish it from any other 

 likely to be confounded with it. It resembles P. Benauxiana d'Orbigny, 

 as figured by him in Paltiontologie Fran^aise, but it expands much more 

 rapidly in width than that species does, is not proportionally so thick, and 

 has a very different posterior marginal outline. In that shell it is the mid- 

 dle of the posterior margin that is most extended, while in ours the greatest 

 extension is much below the middle. 



Position and locality. — Strata of the Cretaceous period ; east of Mount 

 Taylor, one mile south of Pajuate. New Mexico. 



Order DYMYARIA. 

 Family ARCIDJE. 



Genus IDONEAKCA Courad, 1862. 



Idonearca depressa White. 



Plate XVIII, fig. 13 a and 6. 



Shell of moderate size, gibbous, irregularly trapezoidal or subovate in 

 marginal outline ; posterior half of the basal margin nearly straight, broadly 

 rounding upward anteriorly, and sharply rounding the end of the umbonal 

 ridge to meet the posterior margin ; anterior margin regularly rounded up 

 to the hinge-line ; posterior margin long, about equal in length to the full 

 height of the shell, nearly straight or slightly convex, extending obliquely 

 downward and backward ; hinge-line equal to a little more than half the 

 length of the shell ; areas small, narrow, well defined, slightly concave ; 

 beaks very small, depressed, a little incurved ; umbonal ridges very promi- 

 nent, each bounding anteriorly a flattened, three-sided space, along the 

 middle of which there is a radiating, raised line. 



