62 PALEONTOLOGY OF THE UPPER MISSOURI. 



Pleiiroplioriis? Calhouni. 



(Plate II, Fig. 13, a, 6.) 

 Edmondia? Calhoimi, Meek & Hayden, Trans. Albany Inst. IV, March 2d, 1858. 



Shell longitudinally subovate, gibbous over the oblique umbonal slopes. Extremities rather narrowly rounded ; 

 base forming a broad semi-elliptic curve, sometimes nearly straight along the middle ; dorsal side declining rather 

 gradually from the beaks posteriorly, and more abruptly in front. Hinge straight, apparently thickened within, so 

 as to leave on internal casts a slightly impressed lanceolate area, along the cardinal region behind the beaks. 

 Umbones moderately prominent, oblique, and placed a little nearer the anterior extremity than the middle. 

 Anterior adductor muscular impression rather deep, broad ovate, and located near the buccal edge ; scar of the 

 pedal muscle small, oval, deep, and located near the margin of each valve, a little above the impression of the 

 anterior adductor. Posterior muscular impression shallow ; pallial line rather distinct (and undoubtedly simple). 



Length (of an internal cast), 1.45 inch ; height, 0.75 inch ; convexity, 0.80 inch. 



Since first referring this species with doubt to the genus Edmondia, we have 

 satisfied ourselves that it cannot be retained in that genus ; there being no traces 

 left in weU preserved internal casts, of the cardinal appendages for the attachment 

 of the cartilage, as in Edmondia. Although we now place it provisionally in the 

 genus PleuropJiorus, we are not clearly satisfied in regard to its relations to that 

 group ; indeed, so far as we can determine from impressions of the hinge left in 

 the matrix, it does not appear to have been exactly like that of the tj^ical species 

 of that genus. 



This species may be at once distinguished from the last, by its larger size, pro- 

 portionally shorter form, and less prominent internal ridge just behind the anterior 

 adductor scar — as well as by its more prominent umbones, which are also placed 

 farther back. 



Locality and position same as last. (Type 4184.) 



