382 DESCRIPTIONS OF NEW SPECIES OF FOSSILS, 



AVICULA HAYDENI, n. sp. 

 PLATE I. FIG. 5, a, b. 



Shell small, sub-rhomboidal, oblique ; beak small, pointed, slightly elevated above the 

 hinge line ; hinge line straight, less than the length of the shell, and pointed at the 

 posterior extremity, anterior extremity short, rounded; posterior margin obliquely 

 truncate ; no line of demarcation between the wing and body of the shell ; basal 

 margin forming a regular elliptic curve ; surface marked by sharp strong ribs, with 

 sometimes an intermediate smaller one, crossed by faint concentric undulations and 

 parallel fine lines of growth. Length, .3 inch ; height, .22 inch ; hinge line making 

 an angle with the posterior slope of about 129. 



All the specimens we have seen are of the left valve only. The shell is extremely 

 thin and fragile, and preserved only upon portions of one of our specimens. Some 

 individuals show a depressed line along the hinge margin. 



Locality and Position. On the Missouri, near Red Cedar Island, twenty-five miles 

 below Fort Pierre. From division No. 4 of Section. 



LUCINA SUBUNDATA. 

 PLATE I. FIG. 6, a, b. 



Shell sub-orbicular ; length a little greater than height ; beak little elevated, sub- 

 central or nearer the posterior side ; anterior margin broadly rounded, posterior one 

 sloping from the beak and rounded below ; surface with concentric undulations and 

 finer parallel lines, crossed by very minute radiating strise. Length, A inch ; 

 height, .36 inch ; width, .2 inch. 



This shell bears some general resemblance to L. cornuelana of D'Orbigny, ( Terrains 

 Cretaces, p. 116, pi. 281, fig. 3,) but the beaks are much less elevated, the anterior end 

 much broader, and the concentric undulations larger and less uniform. This species 

 strikingly possesses the characters of the genus Lucina, and may readily be distin- 

 guished among the smaller bivalves from this region. 



Locality and Position. Sage Creek. Upper part of division No. 4 of Section. 



CYTHEREA ORBICULATA, n. sp. 



PLATE I. FIG. 7. 

 Shell thick, sub-orbicular ; beak moderately elevated and near the anterior side ; 



