36 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 49. 



generic position is shown by the statement following the description: 

 "None of the specimens retains its operculum. The form of the 

 shell, and especially of the aperture, recalls Rissoina, but the irregular 

 varices, sculpture, and apex are more like Alaba. A certain amount 

 of doubt as to its true zoological position must remain until the 

 operculum is known." 



I wish to express my extreme indebtedness to the Board of Trustees 

 of Amherst College, and particularly to Prof. F. B. Loomis, tlirough 

 whose kind offices it was possible for me to have Prof. C. B. Adams' 

 t3"pes of the Panama species for study, compai-ison and figuring. I 

 am also indebted to Mr. Adolph Jensen, of the Zoological Museum 

 of Copenhagen, for the loan of O. A. L. Morch's types of his Central 

 American forms. 



The illustrations accompanying this report, with few exceptions, 

 are from enlarged photographs made direct from specimens by Mr. 

 T. W. SmilUe, of the United States National Museum, and were 

 retouched by Mrs. E. B. Decker. The exceptions are: 



Carpenter's Rissoina woodwardii, of which I have not seen speci- 

 mens, and of which I give camera lucida figures by Doctor Car- 

 penter. 



Of Rissoina signae, new name, Rissoa insigne DeFohn, Rissoina 



laurae DeFolin and Rissoina zeltneri DeFolin — I give figures copied 



from DeFolin's works because I have been unable to obtain specimens 



of these species. 



ANALYTICAL KEYS. 



KEY TO THE SUBGENERA OF THE WEST AMERICAN RISSOINAS. 



Posterior angle of the aperture simple Rissoina, pp. 38-61. 



Posterior angle of the apertui'e almost constricted off into a 

 separate opening by a projection from the parietal wall, 

 and another apposing from the outer lip Folinia, p. 61. 



KEY TO SPECIES OF WEST AMERICAN RISSOINAS. 



a^. Spiral sculpture present. 



6'. Spiral sculpture on spire and base, 

 c'. Axial sculpture consisting of prominent ribs, 

 c?'. Axial ribs acutely sublamellar; intercostal spaces with cloth- 

 like textured sculpture. 



e\ Adult shell about 5 mm. long firmata, p. 38. 



e^. Adult shell about 3 mm. long excolpa, p. 39. 



d"^. Axial ribs not acutely sublamellar; intercostal spaces without 

 cloth-like textured sculpture. 

 e^. Summit of whorls strongly shouldered. 

 /'. Base evenly curved; without a tumid area or keel on the 

 anterior third . 

 g^. Shell broadly conic. 



h^. Basal sculpture uniform. 

 i^. Intercostal spaces about as wide as the ribs. 



/. Basal cords 15, adult more than 8.5 mm stricta, p. 39. 



j^. Basal cords 13, adult not more than 7.5 mm fortis, p. 40. 



