CHIEFLY OF THE UNITED STATES. 35 



beaks ; cavity of the shell rather shallow and wide ; cavity of the beaks rather 

 shallow and angular ; nacre pale salmon, satin-like and very iridescent. 



Remarks. — Like the preceding, only a single specimen was sent to me by Dr. Kirk. 

 It is closely allied to Nyassaensis herein described. It seems to be a smaller species* 

 with more folds which corrugate in the middle, quite oblique, and with a beautiful 

 rich satin-like pearly nacre, which is very pale in this specimen, but may be darker 

 or even white in others. 



Spatha alata. pi. 12, fig. ol. 



Testa alata, sulcata, triaugulari, compressa, valde inasquilaterali, antice oblique rotuudata, postice obtuse 

 angulata ; valvulis orassiusculis ; natibus parvis, vis promiuulis; epidermiJe olivaoea, nitida, obsolete 

 radiata ; margarita purpurea et valde iridescente. 



Shell winged, sulcata, triangular, compressed, very inequilateral, obliquely rounded 

 before, obtusely angular behind ; valves rather thick ; beaks small, scarcely promi- 

 nent; epidermis olive green, shining, obscurely raj'ed ; nacre purple and very irides- 

 cent. 



Proc. Acad. Nat. Sci., 18G4, p. 109. 



Eah. — Lake Nyassa, Central Africa, John Kirk, M. D., of the Zambezi Expedi- 

 tion. 



My cabinet. 

 Diam. -8, Length 1-8, Breadth o'l inches. 



Shell winged, sulcate, triangular, compressed, flattish on the sides, very inequilat- 

 eral, obliquely rounded before and obtusely angular behind ; substance of the shell 

 rather thick ; beaks small, scarcely prominent ; ligament long, rather thin and dark 

 brown ; epidermis olive green, shining, with distant lines of growth and obscurely 

 rayed ; umbonial slope slightly raised and obtusely angular ; posterior slope raised 

 into rather a high wing; anterior cicatrices distinct, large and well impressed; pos- 

 terior cicatrices distinct, large and well impressed ; dorsal cicatrices form a line across 

 the centre of the cavity of the beaks and are well impressed ; cavity of the shell 

 shallow and wide ; cavity of the beaks scarcely perceptible ; nacre purple, paling 

 towards the margin and very iridescent. 



Remarks. — This species difi'ers much from any I have seen of the genus in having 

 a well developed wing which rises from the point of the beaks to the outer edge, 

 taking in this respect somewhat the appearance of some of the Uaiones, viz. : delphi- 

 nuUs, Morelet, and Icevissimus, (nobis.) Its greatest perpendicular line is from the 

 outer angle of the wing to the base. It narrows down towards the anterior margin. 

 In this specimen the rays are greenish and stronger on the anterior third of the disks. 

 The epidermis is so thin that the iridescence of the nacre is perceptible through it. 

 This specimen is slightly emarginate at the base, but this may not be a permanent 



