UNITED STATES AND ARCTIC AMERICA. 199 



initial and rough ; lateral teeth thick, corrugate, short and somewhat curved ; nacre 

 white and iridescent. 



. Proe. Acad. Nat. Sci., 1861, p. 392. 



Hal.— Dallas, Texas, Prof. C. G. Forshey. 

 . My cabinet and cabinet of Prof. Forshey. 

 Diam. 1, Length 13, Breadth 1-4 inch. 



Shell smooth, subtriangular, very much inflated, swollen at the umbones, obtusely 

 angular behind and rounded before, nearly equilateral ; substance of the shell thick, 

 slightly thicker before; beaks very prominent, incurved, covered at the points with 

 small folds ; ligament short, thick and light brown ; epidermis brownish olive, ob- 

 scurely radiate, with three very distant marks of growth, striate and dull below ; 

 umbonial slope well raised and angular; posterior slope flattened, wide, cordate, 

 with two obscure lines on each valve from the beaks to the margin ; cardinal teeth 

 rather small, subpyramidal and rough; lateral teeth thick, corrugate, short and 

 somewhat curved ; anterior cicatrices rather small, distinct and well impressed ; 

 posterior cicatrices distinct and moderately well impressed ; dorsal cicatrices placed 

 over the centre of the cavity of the beaks ; cavity of the shell oW) and rounded ; 

 cavity of the beaks rather deep and angular ; nacre white and iridescent. 



Memories. — The diagnosis is made from a single specimen, which, however, is 

 entirely perfect. It belongs to that group of which trigonus (nobis) may be con- 

 sidered the type, but it differs in being rounder, even more inflated and in the 

 character of the undulations on the beaks. These undulations are remarkably 

 close in Riddellii, while in trigonus they are few and follow down the angle of the 

 umbonial slope for a short distance. There are a few obscure thin rays on the 

 specimen before me. It is likely that generally they will be found without any. 

 The lines of growth are broad, very distant and well defined. The beaks are shin- 

 ing, but the remainder of the disk is dull and striate. 



Unio contiguus. PI. 28, fig. 268. 



Testa laBvi, late elliptica, inflata, insequilaterali, postice obtuse angulata, antice rotuudata ; valvulis 

 subcrassis, antice erassioribus ; natibus subproniinentibus ; epidemiide tenebroso-fusea, valde 

 radiata ; dentibus cardinalibus subgrandis, compressis, crermlatis ; lateralibus longis. lamellatis 

 subrectisque ; margarita 4 salmonis colore tincta et valde iridescente. 



Shell smooth, widely elliptical, inflated, inequilateral, obtusely angular behind, 

 rounded before ; valves rather thick, thicker before ; beaks rather prominent ; epi- 

 dermis dark brown, very much rayed ; cardinal teeth rather large, compressed and 

 crenulate; lateral teeth long, lamellar and nearly straight; nacre salmon color and 

 very iridescent. 



Proc. Acad. Nat. Sci., 1861, p. 392. 



51 



