PALEONTOLOGY. 475 



Genus ALLORISMA King. 

 ALLORISMA ANDREWSI. 

 Plate X, fig. 6. 

 Allorisma Andrezvsi Whitf., Ann. N. Y. Acad. Sci., 1882, p. 222. 



Shell of medium size or smaller, transversely elliptical in outline; the length 

 being about twice the height, and the thickness a little more than two-thirds the 

 height. Valves ventricose, most rotund a little in advance of the middle and along 

 the umbonal ridge, and wedge-shaped posteriorly, as seen in a cardinal view ; beaks 

 of moderate size slightly projecting above the hinge-line, incurved, directed anter- 

 iorly, and situated at about one-sixth of the entire length from the anterior end. 

 Cardinal line straight or appearing slightly concave, extending about three-fourths 

 of the length of the shell from the beaks backward, and bordered by a proportion- 

 ally large and wide escutcheon. Anterior end short, sloping forward from between 

 the beaks, at about an angle of forty-five degrees to the hinge-line, to near the mid- 

 dle of the height of the shell, and then abruptly rounding backward into the some- 

 what regularly convex basal margin. Posterior end broadly rounded from the point 

 of the umbonal ridge to the extremity of the cardinal line. Anterior end of the 

 shell characterized by a very small lunule. Surface of the shell marked by several 

 strong concentric undulations or folds, which are simple, and regularly increase in 

 size and strength to near the full size of the shell; but near the outer margin of the 

 valves, in the specimen figured, the}' are smaller and doubled by the interpolation 

 of an intermediate rib. The undulations are crossed obliquely from the beak to 

 the basal margin, just posterior to the middle, by a narrow, almost imperceptible 

 sulcus, and along the crest of the umbonal ridge by a line of low-convex and faintly- 

 marked nodes, one on the surface of each undulation ; the posterior umbonal slope 

 is also marked, immediately below the margin of the escutcheon, by a slightly con- 

 cave sulcus, across which the undulations are more faintly marked than below. 



The species is closely allied to Allorisma clavata McChesney, and 

 was at first supposed to be identical; but on comparison, it shows so 

 many points of difference that it became necessary to consider it as a dis- 

 tinct species. 



Formation and Locality. — In limestone of the age of the Chester group 

 (or Chester and St. Louis combined), at Newtonville, Ohio. Collected by 

 K. B. Andrews, to whom the species is dedicated. 



ALLORISMA MAXVILLENSTS. 



Plate X, figs. 7 and 8. 



Allorisma Maxvillensis Whitf., Ann. N. Y. Acad. Sci., 1882, p. 222. 



Shell small, the specimen used being a little less than one inch in length, and 

 the height less than half the length. Form of the shell transversely elongate, 

 and cylindrically oval, the cardinal and basal margins parallel and very slightly 

 curved, and the extremities very nearly equally rounded; beaks small, inrolled, 

 barely projecting above the cardinal line, and situated at about one-fourth of the 

 entire length from the anterior end. Body of the shell very evenly and highly 

 rounded from the cardinal to the basal margins, and almost as convex posteriorly as 

 in front. Umbonal ridge scarcely perceptible, and the umbonal slope convex; 

 escutcheon and lunule not defined ; anterior slope abruptly rounded. Surface of 

 the shell marked by faint concentric undulations of unequal strength, but most 

 strongly marked on the posterior end and on the umbonal slope. 



