30 NEW UNIONID^, MELANID^, ETC., 



behind and rounded before; valves thin, somewhat translucent; beaks somewhat 

 elevated, granulate at the tips; epidermis yellowish green or darkish, without rays; 

 nacre bluish white and iridescent. 



Proc. Acad. Nat. Sci., 1863, p. 190. 



Hah. — Lake Winnipeg, at the mouth of the Saskatchewan River, R. Kennicott. 



My cabinet and cabinets of Smithsonian Institution and Academy of Natural 

 Sciences. 

 Diam. -6, Length 1-1, Breadth 1-9 inches. 



Shell smooth, subelliptical, slightly inflated, inequilateral, obtusely angular behind 

 and rounded before; substance of the shell thin, semi-transparent; beaks somewhat 

 raised, granulate at the tips ; ligament rather large, light brown ; epidermis yellowish 

 green or dark approaching to mouse color, with four or five rather distant, distinct 

 marks of growth ; umbonial slope slightly raised and rounded ; posterior slope rather 

 contracted, slightly carinate, with three indistinct greenish lines on each valve from • 

 the beaks to the posterior margin ; anterior cicatrices confluent, large and indistinct ; 

 posterior cicatrices confluent, large and indistinct ; dorsal cicatrices placed in the 

 upper part of the centre of the cavity of the beaks ; cavity of the shell rather shallow 

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

 and iridescent. 



Remarks. — Quite a number of this species were brought by Mr. Kennicott, but 

 many were broken in the carriage for so long a distance. None were brought in 

 alcohol, so that the soft parts were not observed. This species is allied to Kennicottii 

 and Simpsoniana, (nobis), but differs in the outline, color of the epidermis, and has 

 fewer and more distant marks of growth. The beaks of the three species are very 

 much the same, having a double row of small undulations at the tips, which undula- 

 tions rise into granules. I name this species after the Hon. Mr. Dallas, Governor of 

 the Hudson's Bay Company, and successor of Sir George Simpson, who has liberally 

 and efficiently promoted the objects of Mr. Kennicott's expedition to Arctic America, 

 and has been interested in the increase of our knowledge of the natural history and 

 resources of the country under his supervision, extending facilities at all times to the 

 operations of the Smithsonian Institution. 



MONOCONDYLCEA COMPRESSA. PI. 11, fig. 29. 



Testa lasvi, transversa, valde insequilaterali, valde compressa, ad latere planulata, postice angulata, antice 

 rotundata ; valvulis pertenuibus ; natibus vix prominentibus, ad apices undulatis ; epidermide micante 

 olivacea, obsolete radiata ; deute cardinali in destra valvula solum et minimus; margarita albida et 

 valde ii'ideseente. 



Shell smooth, transverse, very inequilateral, very much compressed, flattened on 

 the sides, angular behind and rounded before ; valves very thin ; beaks scarcely 



