1897.] NEW YORK ACADEMY OF SCIENCES. 265 



Allorisma walkeri sp. nov. 

 PI. XX., figs. 6-7. 



Shell longitudinally sub-elliptical in outline, with the dorsal 

 and ventral margins sub-parallel. Yalves moderatel}' convex, 

 the greatest convexitj' being about the middle, somewhat com- 

 pressed posteriorly. Umbonal ridge ill-defined or obsolete, 

 often fiattened, giving the appearance of two indistinct ridges 

 extending from the beak to the postero-ventral margin. Beaks 

 of moderate size, projecting slightly above the hinge-line, in- 

 curved, directed anteriorly and situated at about one-fourth the 

 length of the shell from the anterior end. 



Cardinal line straight, extending three-fourths the length of 

 the shell backward from the beaks, anterior end short, sloping 

 forward from between the beaks at an angle of forty-five degrees 

 or less, to about the middle of the height of the shell, then 

 gently curving downward to form the sub-semi-circular anterior 

 margin of the shell. Ventral margin nearly straight or slightly 

 arcuate from a point directly below the beaks to one-half the 

 distance from this point to the posterior extremit}'; posteriori}^ 

 the margin curves regularly upward and then forward, forming 

 the nearly semi-circular posterior margin. 



Anterior end of the shell with a small but well defined lunule. 

 Surface ornamented with strong, irregular, concentric undula- 

 tions. In some specimens a broad, shallow, ill-defined, almost 

 imperceptible sulcus passes from the beak to about the middle 

 of the ventral margin ; in other specimens this sulcus is obso- 

 lete. 



This species is allied to A. maxville77sis, Wh'it^eld,* from the 

 Maxville limestone of Ohio, but is much larger and the beaks 

 are situated farther from the anterior end of the shell. 



The species is named in honor of Mr. Geo. C. Walker, the 

 founder of Walker Museum, University of Chicago. 



Allorisma arkansana sp. nov. 



PI. XX., figs. 4-5. 



Shell longitudinally sub-elliptical in outline. Equivalve, the 

 valves rather strongh' convex in the umbonal region, compressed 

 posteriori}'. Umbonal ridge ill-defined, rounded, becoming 

 nearly obsolete posteriorly. Beaks prominent, incurved, i)ro- 

 jecting considerably above the hinge-line, situated at a point 



♦Ann. N. Y. Acad. Sci., Vol. 5, p. 588, PI. 14, figs. 7, 8, and Geol. Surv. Ohio, Vol. 

 7, p, 476, PL 10, figs. 7, 8. 



