TROCHUS. 9 



simple, arcuate and spread upon the base at its insertion, below 

 tuberculate, swollen, ending in a tooth, or simple ; outer lip smooth 

 or lirate within. 



Subgenus MONODONTA Lam. 



Shell Uirbinate-conic, very heavy, thick, solid; columella strong, 

 cylindrical, bulging or more or less toothed near or at the base ; 

 aperture as high as wide. 



Section MONODONTA (restricted). 



Shell smooth or spirally ridged ; outer lip plicate within ; colu- 

 mella short porcellanous, terminating abruptly in a tooth, between 

 which and the basal margin there is a square notch or channel. 

 Type, M. labio L. Indian 0. 



Section AUSTROCOCHLEA Fischer. 



Shell like Monodonta s. sir., but columella only slightly toothed 

 at the base, not notched ; outer lip lirate within. Type, M. constricta 

 Lam. Australasia. 



Section OSILINUS Philippi. 



Shell smooth or obsoletely spirally grooved ; outer lip smooth 

 within ; columella swollen and convex in the middle, pearly, con- 

 tinuous below with the basal lip. Type, M. turbinata Born. 



Mediterranean Sea. 



Subgenus DILOMA Philippi. 



Shell globose or depressed-conic ; aperture large, very oblique ; 

 columella not prominent, flattened, not cylindrical, generally con- 

 cave, arcuate, and slightly denticulate at the base or smooth. 



Section DILOMA (restricted). 



Shell globose, depressed or conic, imperforate, black ; smooth or 

 spirally grooved ; columella wide, concave, porcellanous; lip mar- 

 gined with an iridescent band which extends across the parietal 

 wall. Type. M. nigerrima (Gmel.) Phil. 



W. Coast S. America. 



Section XEODILOMA Fischer. 



Similar to the preceding, but without the parietal band of irides- 

 cent nacre ; surface smooth, grooved or lirate ; unicolored, spotted 

 or tessellated ; columella with one or two denticles at base, or 

 smooth. Type, M. sethiops Gmel. Australasia, 



