164 CHLOROSTOMA. 



Zoologie, t. 51 (1832) (no description). H. and A. AD., Genera, i, 

 p. 426. 



The genus Chlorostoma replaces on American shores the old 

 world genus Gibbula. The more prominent shell characters are the 

 callous which is continued from the columella partly around (or 

 even covering) the umbilicus, and the bi- or tri-dentate base of the 

 columella. The dentition is peculiar, and at once separates these 

 shells from all other Trochidse. The rhachidian tooth is broader 

 than long, the reflected portion extending nearly its whole breadth, 

 narrowly reflected and with irregularly denticulated edge. The 

 laterals number 5 on each side. The uncini have either denticulate 

 or simple cusps (pi. 50, fig. 10, C. fanebrale; fig. 13, Omphalhi* 

 excavatus). 



The distribution of the species of Chlorostoma into sections is 

 difficult on account of the coalescence of characters throughout 

 the group ; and many species go equally well in Chlorostoma or 

 Omphalius; in general, Omphalius includes umbilicate species with 

 spirally lirate or granose whorls, but some undoubted Omphalius 

 are smooth, and some granose species are imperforate. Correspond- 

 ing variations occur in the large conical typical Chlorostoma ; so 

 that no sharp line can be drawn. C. pellisserpentis, the only species 

 of Tegula (Less.) Gray, is perhaps distinct enough to have a sec- 

 tional name. 



Oriental Species. 



The following key, slightly modified from Lischke, will usually 

 guide one to the species of Chlorostoma inhabiting Japanese waters : 



A. Umbilicus covered. 



1. Longitudinally plicate : 



a. Large, folds strong, base spirally lirate, C. argyrostoma. 



b. Smaller ; folds fine, close ; base smooth, C. distinguenda. 



2. Without longitudinal folds : 



a. Coal-black, with orange apex, C. nigricolor. 



b. Olive-colored, with lead-colored apex, C. lischkei. 



B. Openly umbilicate. 



1 . Periphery bluntly angled ; base plano-convex or flat : 



a. Elevated ; with broad low folds or smooth, C. rustica. 



b. Depressed ; folds very narrow and close, C. nigerrima. 



2. Periphery acutely angled ; base plano-concave. 



