Description of a New Species of Spirifer. 29 



prominent near the center; beak somewhat prominent, only 

 slightly incurved; area relatively wide, directed backward; 

 middle of valve occupied by an indistinct mesial fold that 

 becomes entirely obsolete near the umbo. 



Ventral valve sub-triangular as seen from the side, a little 

 more convex than the dorsal, arcuate from beak to margin 

 with curvature most abrupt in the umbonal region: mesial 

 sinus shallow, imperfectly defined in the anterior and middle 

 portions of the valve and fading out toward the beak; hinge 

 area wide, its surface forming an isosceles triangle with sides 

 equal to two thirds the base; the area is strongly concave, 

 curving downward, backward, then upward through an arc 

 of slightly more than ninety degrees; foramen large, three 

 fourths as wide as long, open to the beak. 



Surface marked by from eight to ten obscure plications on 

 each side of the mesial fold and sinus. The plications are only 

 visible on the anterior and the antero-lateral portions of the 

 valve, fadmg out on the umbonal and postero-lateral portions. 

 The finer markings, if there were any, have been removed by 

 weathering from the onl}^ specimen of this species that has fal- 

 len under observation. There are a few concentric lines of 

 growth on both valves. 



Length 1.3 inches, width 1.2, thickness 0.65 inches. 



The type specimen was found free in a small ravine cut in 

 the Hamilton limestone and shales near Iowa Cit}', Iowa. 

 There is every reason to believe that it was derived from the 

 same beds that furnished the other species of fossils with 

 which it was associated. 



The short hinge, triangular outline, elliptical dorsal valve, 

 wide concave area, prominent incurved beak, obscure fold and 

 sinus, and rudimentary plications constitute a combination of 

 characters whereby tliis species may be distinguished from all 

 other described forms. 



