186 BULLETIN 192, UNITED STATES NATIONAL MUSEUM 



elongate-conic. The axial sculpture may consist of strong ribs or range 

 from these to slender, almost lamellar riblets, or it may be reduced to 

 incremental lines. There is also a wide range of strength in the develop- 

 ment of the spiral sculpture, which may be confined to the umbilicus 

 or may cover the entire shell. Breathing devices are present in some 

 groups and absent in others. The chief character of the subfamily, 

 however, is found in the operculum, which consists of a thin, simple 

 chondroid basal plate of several whorls, the outer edge of which may 

 be faintly upturned to form a very fragile, low, slender lamella, suggest- 

 ing the starting point of the subfamily Adamsiellinae. This is, however, 

 usually soon brushed away, leaving the operculum as a plain plate. The 

 operculum has a deposit of fine calcareous granules, which may be very 

 slight or fairly strong, depending upon the species in question. 

 Type genus : Chondropoma Pfeififer. 



Genus CHONDROPOMA PfeiflFer 



1847. CJwndropojna Pfeiffer, Zeitschr. Malak., vol. 6, p. 109. 



Shell ranging in form from turbinate to elongate-conic; the sculpture 

 in varying intensity may consist of axial ribs only or of axial ribs and 

 finer axial threads or of axial ribs and spiral threads. All, even those 

 without spiral sculpture on spire and base, have spiral threads on the 

 umbilical wall. No special breathing device is developed in the members 

 of this genus. The operculum is simple; that is, it consists of a chondroid 

 plate made up of a varying number of whorls, the outer thin edges of 

 which are sometimes faintly upturned to form a suggestion of an ob- 

 solete lamella. The outer surface of the operculum has a deposit of 

 calcareous granules, which is usually very slight but in some species 

 rather pronounced. In no instance is this entirely absent. The position 

 of the opercular nucleus, whether excentric or subcentral, depends upon 

 the shape of the aperture. 



Type species : Cydostoma sagra Orbigny = Chondropouia (Chon- 

 dropoma) pictum sagra (Orbigny). Selected by Petit in i85o. 



KEY TO THE RAHAMAN SUBGENERA OF CHONDROPOMA 



Axial and spiral sculpture absent Levipoma 



Axial and spiral sculpture not absent. 

 Summit of whorls tufted. 



Truncated shell pupoid Chondrops 



Truncated shell ovate Chondropomorus 



Summit of whorls not tufted. 



Junction of axial and spiral sculpture forming short 



cusps Chondropomisca 



Junction of axial and spiral sculpture not forming short 



cusps Chondropoma 



Levipoma, new subgenus 



Shell elongate-conic with the spiral sculpture absent and the axial 



