588 Contributions to Invertebrate Palaeontology. 



Genus ALLORISIflA King. 



Allorisiiia Aiidrewsi. 



Plate XIV, fig. 6. 



Allorisma Andrewsi Wh\iL, 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 anteriorly, 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 proportionally large and wide escut- 

 cheon. Anterior end short, sloping forward from between the beaks, at about 

 an angle of forty-five degrees to the hinge-line, to near the middle of the 

 height of the shell, and then abruptly rounding backward into the somewhat 

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

 of the umbonal ridge to the extremity of tlie 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 regu- 

 larly 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, they 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 imdulation ; the posterior umbonal slope is also marked, im- 

 mediately below the margin of the escutcheon, by a slightly concave sulcus, 

 across which the undulations are more faintly marked than below. 



The species is closely allied to Allorisma clavata McChesney, and 

 was at iirst supposed to be identical ; hut on comparison, it shows 

 so many points of difference that it became necessary to consider it 

 as a distinct species. 



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

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

 Collected by Prof. E. B. Andrews, to whom the species is dedicated. 



Allorisma Iflaxvillensis. 



Plate XIV, 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 



