SHUMARDELLA 223 



the deltidial plates not observed ; plications few in number, "broad, low, 

 rounded, two, or more rarely three, originating on or just in front of the 

 umbonal region and occupy the bottom of the sinus, on each lateral slope 

 of the valve two plications are usually present which are fainter than 

 those of the sinus and originate closer to the margin. Internally the teeth 

 are supported by a pair of dental plates which quickly become obsolete 

 in front of the articulation of the hinge. 



Brachial valve gibbous, the greatest depth near the front margin in 

 adult examples, the surface usually slopes along the median line from the 

 point of greatest convexity to the beak with a nearly regular curvature, 

 becoming a little more convex near the beak, anteriorly there is often a 

 short but somewhat abrupt slope to the margin, towards the lateral mar- 

 gins the surface curves at first gently and then more abruptly from the 

 median line, and towards the beak on either side there is a distinct and 

 sometimes rather deep lateral concave region ; mesial fold scarcely or not 

 at all differentiated in the posterior half of the valve, usually becoming 

 more prominent anteriorly; beak incurved beneath that of the opposite 

 valve ; plications similar in form and number to those of the opposite 

 valve, the three or rarely four of the mesial fold alternate with those of 

 the sinus and are the strongest ones on the valves, those of the lateral 

 slopes often nearly or quite obsolete. Internally there is no cardinal pro- 

 cess, a thin and rather short median septum is present supporting the in- 

 itial portion of the hinge-plate, at the apex of the beak this septum is not 

 divided but anteriorly it is split into two parts, one of which supports 

 each side of the hinge-plate, but the base of the hinge-plate is solid and 

 roofs over the cavity between the two divisions of the septa, so that they 

 do not form an open crural cavity such as is present in the genus 

 Camarotwchia, anteriorly the hinge-plate is divided and at the point of 

 articulation of the hinge the two divisions are separate and the median 

 septum is so much reduced that it no longer supports them. The crura ex- 

 tend from the anterior extremities of the divided hinge-plate into the 

 cavity of the shell. 



The surface of both valves is marked by fine, regular, concentric stria?, 

 and usually by several stronger, irregularly developed, concentric lines of 

 growth towards the front. 



Remarks. This species has been frequently misinterpreted. Every 

 published illustration to which the name has been applied, save only the 

 original one by Shumard, is incorrectly identified, and even this author 

 has represented two different species. The name missouriensis is here re- 

 tained for the larger form illustrated by Shumard, and the one with 

 which his description more fully agrees. The species is restricted to the 

 Chouteau limestone fauna and is most commonly represented in the typi- 

 cal Chouteau limestone in central Missouri. 



