58 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 58. 



I'. Shell helicoid Subgenus Tvdorellata. 



J^ Umbilicus bounded by a spiral cord Group interstitialis . 



J-. Umbilicus not bounded by a spiral cord. . . Group mtricovmvi. 

 P. Shell not helicoid. 

 J^ Shell elongate-ovate — elongate-conic. 



K'. Spiral sculpture absent Subgenus Colobostyhts. 



K^. Spiral sculpture not absent. 



L'. Spiral sculpture confined to the umbilicus 



Subgenus Tvdorisra. 



M'. Shell elongate-ovate Group albus. 



W. Shell not elongate-ovate. 



N. Shell elongate-conic Group chiapensis. 



L^. Spiral sculpture not confined to the umbilicus-. 

 M'. Axial ribs not rendered articulated by the spiral 



sculpture Subgenus Tvdora. 



N*. Kiblets tufted at the summit. 



O'. Shell elongate-ovate Group rangelinum. 



O^. Shell not elongate-ovate. 



P. Shell elongate-conic Group abbotti. 



W. Riblets not tufted at the summit. 



O'. Suture channeled Group habichi. 



0-. Suture not channeled Group mcgacheila. 



W. Axial ribs rendered articulated by the spiral sculpture 



Subgenus Tudor ops. 

 N'. Shell ovate-conic. 



O^ Axial ribs gathered into tufts at the summit. 



Group banksiana. 

 0'\ Axial ribs not gathered into tufts at the summit. 



Group xanthosioma. 



N^. Shell elongate-conic Group undosum. 



H^. Breathing notch, slit, pore, or siphon present Genus Eutudora. 



I'. Axial sculpture almost obsolete. 



J'. Breathing notch present Subgenus Eutudorisca. 



Z^. Breathing notch not present. 



K^ Breathing slit present Subgenus Eutudorella. 



K^. Breathing slit not present. 



L' . Breathing pore present Subgenus Eutudora. 



P. Axial sculpture not almost obsolete. 



J. Axial sculpture sublamellar Subgenus Eutudorops. 



K'. Breathing pore without external siphon . . Group azucarcnsis. 



K2. Breathing pore with external siphon Group torquatum. 



G^. Lamellae not flat. 



H. Lamellae convex Genus Rav}sden%a. 



F^. Lamellae not marked by feeble oblique riblets. 



G. Lamellae marked by strong oblique riblets Genus Diplopoma. 



H'. Outer surface of operculum not convex in the center 



Subgenus Diplopoma. 



I'. Shell ovate-conic Group retrorsuum. 



P. Shell not ovate-conic. 



J. Shell elongate-conic Group architectoiiicuvi. 



H^. Outer surface of operculum convex in the center 



Subgenus Janiuicia. 



