86 [April, 



Arca (cucull/Ea) cordata. Shell thick and strong, subovate, somewhat 

 triangular or cordiform, very ventricose, abruptly rounded before, obliquely 

 subtruncate behind, and forming a broad curve below; uniboncs gibbous, much 

 elevated, incurved, located near the anterior end ; ligamentary area very short, 

 deeply excavated, grooved and finely striated longitudinally ; surface marked 

 with strong imbricating lines of growth and indistinct radiating costffi. Length 

 2-03 inches; breadth 1-77 inches ; height 1-9G inches. Length of ligamentary 

 area 1-25 inches ; breadth of do. -30 inches. 



This species, in its thickness and general appearance, approaches Cucullcea 

 Ifebrascensis, (Owen,) but may be distinguished by its shorter buccal extremity, 

 more elevated, approximate and incurved beaks, and especially by its much 

 shorter and narrower ligamentary area. There is in our specimen a distinct 

 sulcus starting from before the beak of each valve, and extending obliquely 

 downwards to a point a little behind the middle of the shell, where it dies out 

 before reaching the border. When viewed on either end, this shell presents a 

 beautiful cordate form. 



Locality and position. Moreau Trading Post, No. 5 of series. 



Arca (cucdll/Ea) Shumardi. Shell oval, ovate, somewhat globose, rather 

 thin, obliquely subtruncate behind, rounded before, and forming an elliptic curve 

 below ; umbones very gibbous, oblique, moderately elevated, incurved, and 

 located a little in advance of the centre ; surface ornamented by distinct lines of 

 growth, crossed by numerous, rather indistinct radiating costaj ; ligamentary 

 area straight, narrow, moderately excavated, grooved and striated longitu- 

 dinalljr ; hinge having in each valve three or four transversely striated, oblique, 

 lateral teeth on each side, and small, irregular, intermediate transverse ones in 

 the centre ; border smooth. Length (specimen about four times the medium 

 size) 1'67 inches; breadth 1-28 inches; height 1-40 inches. 



This is one of the most abundant and beautiful bivalves found in the creta- 

 ceous rocks of the upper Missouri country. It varies much in form, some of the 

 specimens being more nearly of an ovate form, in consequence of the anal region 

 being extended, and the beaks more oblique, while others are shorter and more 

 rotund. These two forms are quite well enough marked to constitute a specific 

 distinction, if they were not connected by numerous intermediate ones. As these 

 differences are not due to age or size, we are inclined to regard them as sexual. 

 In old shells the radiating costas are usually obsolete. In some specimens a 

 single raised line maj' be seen passing from behind the beak of the right valve 

 to the posterior border, following the direction of the raised edge of the poste- 

 rior muscular impression. This is rarely seen on the left valve. We dedicate 

 the species to Dr. B. F. Shumard, of St. Louis, Missouri. 



Locality and position. Same as last. 



Mytilus attexuatus. Shell much elongated, slightly arcuate ; extremities 

 compressed and rounded ; anterior end narrow ; beaks nearly terminal ; surface 

 (of cast) faintly marked with lines of growth. Length 1-90 inches ; breadth -50 

 inch ; height -50 inch. 



From M. Galpiniamts and M. Meckii, (Evans and Shumard,) this species may 

 be known by its much greater proportional length and less gibbous beaks. 

 Having only seen casts, we know nothing of its surface markings. 



Locality and position. Same as last. 



Aaicula? fibrosa. Shell ovate, subtriangular, very oblique, pointed at the 

 beaks, somewhat rounded below; beaks small, acute, placed at the anterior ex- 

 tremity, and scarcely rising above the hinge ; right and left valves alike, con- 

 vexly arched from the beaks to the base ; posterior wing somewhat flattened 

 and apparently broadly rounded ; surAxce ornamented by strong, rounded, radi- 

 ating plications, which bifurcate very irregularly, and are crossed bj' strong 

 concentric undulations, so as to give the surface, which is otherwise smooth, a 

 subnodose appearance. Some of our specimens must have been as much as two 

 inches in length when perfect. 



