56 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 58. 



I'. Axial and spiral threads subequal. 



J'. Shell ovate, peristome not expanded Gmuy) semilahre. 



P. Shell not ovate, peristome expanded Group irradians. 



P. Axial and spiral threads not subequal. 

 J. Spiral threads stronger than the axial riblets. 



K'. Shell thin, translucent with interrupted color bands 



Group pictum. 



K^. Shell solid opaque without color bands Group obesvm. 



€"2. Breathing slit or pore present Genus Chondrothyra. 



W. Breathing pore connected with the edge of the parietal wall by a slit 



Subgenus Chondrothyrium. 

 T>^. Breathing pore not connected with the edge ot the parietal wall by a slit. 

 E'. Shell turbinate, spiral scupture absent except in the umbilicus 



Subgenus Chondrothyroma. 

 ¥J. Shell not turbinate, spiral sculpture present everywhere. 



F'. Peristome continuous in one plane Subgenus Chondrothyra. 



G\ Shell ovate conic Group egregium. 



G^. Shell elongate conic Group canaliculata. 



F^. Peristome not continuous in one plane Subgenus Chondrothyrdes. 



GK Shell ovate-conic Group shuUleworthi. 



G^. Shell elongate conic Group sinuosa. 



B2, Basal plate with ribletlike ro-enforcements Subfamily Rhytidopominae. 



C. Calcareous deposit covering the full width of the whorls. 



Genus Rhytidothyra, 

 O. Calcareous deposit not covering the full width of the whorls. 



D'. Calcareous deposits covering only a portion of the width of each whorl. 

 E'. Outer edge of the ribs fused to form a weak lamella. 



F'. Shell without breathing siphon Genus Farachondria. 



G^. Spiral sculpture absent Subgenus Parachondrisca. 



G^. Spiral sculpture not absent. 

 H^ Spiral sculpture confined to umbilicus.. -Subgenus Parachondrdla. 

 IP. Spiral sculpture not confined to umbilicus. 



I'. Spiral sculpture strong not giving an articulate appearance to 



the axial ribs Subgenus Parachondria. 



J' . Peristome expanded Group fascia. 



P. Peristome not expanded Group dentilobata. 



P. Spiral sculpture not strong, giving an articulate appearance to 



the axial ribs Subgenus Parachondrops. 



F2. Shell with breathing siphon Genus OpisthosipJion . 



GK Spiral sculpture confined to the umbilicus . Subgenus Opisthosiphona. 



W. Shell turbinate Group doJli. 



H2. Shell not turbinate. 



I. Shell conic Group inoreletixma. 



G^. Spiral sculpture not confined to the umbilicus. 



Subgenus Opbthosiplwn, 

 W. Outer edge of ribs fused to form a strong lamella. 



F'. Riblets joining the outer and inner lamella, not a.s high as the lamella. 

 G'. Outer lamella in adult shells reflected inward over the whorls. 



Genus Xenopoma. 

 G^. Outer lamella in adult shells not reflected inward over the whorls. 



Genus Ehytidopoma. 

 V'\ Riblets joining the outer and inner lamella a.s high as the lamellae. 



Genus Torrella. 

 G'. Breathing siphon reflected into and closing the umbilicus. 



Subgenus Torrella. 



