UNITED STATES AND ARCTIC AMERICA. 211 



slightly thicker before ; ' beaks slightly prominent, rugosely undulate at the tips ; 

 ligament rather small and light brown; epidermis yellowish, shining, with green 

 rays rather closely set on the posterior part, with distant marks of growth ; umbo- 

 nial slope raised, inflated and rounded ; posterior slope raised into a keel, rather 

 compressed, with two green rays on each valve passing from the beaks to the 

 posterior margin along a wide furrow ; cardinal teeth rather large, oblique, com- 

 pressed, triangular, erect, lamellar and slightly curved upward; anterior cicatrices 

 confluent, indistinct ; posterior cicatrices confluent and very indistinct ; dorsal 

 cicatrices placed over the centre of the cavity of the beaks ; cavity of the shell 

 wide and shallow ; cavity of the beaks rather shallow and subangular ; nacre 

 white, tinted with salmon color above and very iridescent. 



BemarJcs. — The two specimens which are before me were received a long time 

 since from the late President Estabrook. At first, as they came with specimens 

 of M. Holstonia, (nobis,) I considered them as a quadrate variety; but I have 

 never seen, among many of that species since received, any specimen which would 

 connect the two ; I am now satisfied they must be distinct. On comparison, it will 

 be observed that this species is much more square than Holstonia, that it has larger 

 and more lamellar teeth, which are large for the size of the valves; and are curved 

 upwards towards the beak. Both lobes in the left valve are large, and are widely 

 and deeply separated. In the older specimen the rays are distinct nearly over 

 the whole disk, on the umbonial slope they are broader and run together. In 

 smaller and younger specimens the rays are very indistinct. 



Margaritana Gesnerii. PI. 32, fig. 280. 



Testa sulcata, quadrata, inflata, postice obtuse angulata, subequilaterali; valvulis suberassis, antice 

 crassioribus ; natibus subproniinentibus ; epiderniide tenebroso-fusca, micante, obsolete radiata ; 

 dentibus cardinalibus parvis, laevibus, subcoinpressis. in utroque valvulo uuicis; margarita alba. 



Shell sulcate, quadrate, inflated, obtusely angular behind, nearly equilateral ; 

 valves rather thick, thicker before ; beaks somewhat prominent ; epidermis dark 

 brown, shining, obscurely radiate ; cardinal teeth small, rather compressed and 

 single in both valves ; nacre white. 



Proc. Acad. Nat. Sci., 1858. p. 138. 



Hab. — Uphaupee Creek, Alabama, below Columbia, Georgia, Wra, Gesner, M. D. 

 and Mr. Hallenbeck. 



My cabinet and cabinets of Drs. Gesner and Lewis, and Mr. Hallenbeck. 

 Diam. 1-5, Length 2-2, Breadth 3-7 inches. 



Shell sulcate, quadrate, inflated, obtusely angular behind, nearly equilateral; 

 substance of the shell rather thick, thicker before; beaks somewhat prominent; 

 ligament large, rather short and dark brown ; epidermis dark brown, shining, ob- 



54 



