UNITED STATES AND NORTHERN MEXICO. 11 



Shell smooth, pomewhat oblong, coinprep.«ed, flattened at the sides, very inequilateral, 

 somewhat biangular behind, obliquely rounded before; substance of the shell 

 rather thick, very slightly thicker before; beaks slightly prominent, with numerous 

 closely set, concentric undulations at the tips, the undulations being transverse and 

 parallel on the sides ; ligament rather long, somewhat thick and light brown ; epider- 

 mis reddish brown, shining, usually with small green rays, and somewhat distant 

 marks of growth; umbonial slope somewhat raised and obtusely angular; posterior 

 slope rather long and narrow, with two impressed lines on each valve from the beaks 

 to the posterior margin ; cardinal teeth rather large, compressed, erect, crenulate and 

 double in both valves; lateral teeth long, thick, straight and thickened towards the 

 posterior end ; anterior cicatrices distinct, rather large and well impressed ; posterior 

 cicatrices confluent, rather large and slightly impressed ; dorsal cicatrices slightly above 

 the centre of the cavity of the beaks ; cavity of the shell very shallow and wide ; 

 cavity of the beaks very shallow and neaily round; nacre purple and very irides- 

 cent. 



Siift Parts. — Branchial uterus pccupies the whole leaf of the outer Ijranchiiu. 

 Branch ke very wide, very short and nearly straight below, inner ones much the larger, 

 free two-thirds the length of abdominal sack. PaJpi small, ovate, tran.sverse, united 

 only at the upper portion of the posterior edges. Mantle thin, whitish, thickened at 

 the border. Brancluul opening large, with numerous small, brown papillte. Anal 

 openintj rather large, with small brown papilla; on the inner edges. Super-anal open- 

 ing very large and united below. Color of the mass whitish. 



Fmhryonic shell short-pouch shape, clear white and very near to that of phaseolu-i, 

 Ilild. 



Remarks. — This is a very variable species in outline, some being much more trans- 

 verse than others. A very large number was sent to me by Mr. Hallenbeck It is a 

 member of the great complanatus group, but has a tendency to obliqueness, approaching 

 IT. Taomeyi (nobis), that species however is more transversely ovate and smaller ; some 

 of the wider individuals remind one of the lighter colored specimens of 2)«7/a/«9 (nobis), 

 but that shell is wider and more inflated. The young Ilallenlieckii are generally 

 covered with green capillary rays, but some are entirely destitute of them, and are 

 then brown. Some of the adults are greenish, owing to these rays, while others are 

 entirely destitute of them. The undulations of the beaks are usually eight to twelve, 

 and occupy the first growth of the shell, being parallel on the side and obtusely 

 angular on the nmbonial slope. I have great pleasure in dedicating this species to 

 Garret Hallenbeck, Esq., to whom I owe its possession, and who has done so much to 

 promote a knowledge of the mollusca of Georgia. 



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