DALL : AMKRICAN CYCLdSTOMATID^. 209 



layer with the coils separated from the suture by a channel showing 

 the horny basis of the operculum ; a peripheral sulcus present. The 

 radula as in Annularia. 



This is Ctenopoma (Shuttlew.), Pfr., 1856, not of Peters, 1844. 



Genus COLOBOSTYLTJS, Crosse & Fischer, 1888. 



Type : Crjclostoma Jayanum, C. B. Ad. Jamaica. 



Radula as in Annularia. Habit of the shell resembling that of 

 Cyclostoma, auct. (= Ericia). Operculum flat, double-edged, circular 

 or subcircular, few-whorled, slightly concave externally, nucleus 

 subcentral, depressed; calcareous layer smooth or incrementally 

 striate, with no channel at the suture or elevated lamella. 



Genus CHONDROPOMA, Pfeiffer, 1847. 

 Radula with the rhachidian tooth tricuspid. 



Subgenus Chondeopoma, s.s. 



Type : Cyclostoma semtlabris, Lamarck. Haiti. 



Operculum thin, flat, smooth, with no peripheral sulcus, paucispiral, 

 with very excentric nucleus, and whorls rapidly enlarging ; the 

 calcareous layer reduced to a very thin layer of minute superficial 

 granules, or absent entirely. C. pidurn, Pfr., is similar. 



Subgenus Tudora, Gray, 1850. 



Type: T. megacheila (Pot. & Mich.). Curasao. 



Operculum few-whorled, with an excentric nucleus and duplex 

 periphery, the calcareous layer flat, with incremental strise, not 

 channelled at the suture. 



Subgenus nov. Paeachondeia, Dall. 



Type : Turho fascia, Wood. Jamaica. 



Operculum flat, thin, with no peripheral sulcus, with a subcentral 

 nucleus, and few, rapidly enlarging whorls ; calcareous layer appressed, 

 thin, obliquely striate, with a narrow smooth channel at the suture. 



This is Cistula (Humphrey, MS.), Sowerby, 1847, not of Say, 1825. 



1 2 



Genus nov. OPISTHOSIPHON, Dall. 



Type: Chondropoma Bahamense, Sh. (Figs. 1, 2.) Bahamas. 



Shell with the habit of Chondropoma dentatiim, Say, but in the 

 adult with a tubular projection behind and distinct from the outer lip 

 and the posterior angle of the aperture, but communicating with the 

 lumen of the whorl. Operculum as in Rhytidopoma, but thinner. 



