14 Mr. W. H. Benson on the Genus Opisthoporus. 



the outermost lamina, or whorl, causes the singular pulley-like 

 aj)pcarance of the edge. 



The abnormal construction of this operculum fully entitles the 

 form to distinction on the same grounds as those on which the 

 other genera of the Cyclostomacea rest. I do not hesitate, there- 

 fore, to separate it, under the designation some time since pro- 

 posed in my correspondence with Dr. L. PfeifFer, on a view of 

 the species brought home by Lieut. W. Taylor, which subse- 

 quently appear to have passed into the hands of Mr. Cuming ; 

 carefully guarding it from the intrusion of other tubuliferous 

 shells, belonging to Alycceus and Pterocyclos, which Albers has 

 proposed to associate with it in a paper published in the ' Zeit- 

 schrift fiir Malakozoologie ' for 1852. Dr. PfeifFer opposed that 

 view, and, while objecting to the separation of O. Taylorianus 

 from CyclotuSj showed that Albers' scheme would unite shells 

 belonging to several distinct types under the name which I had 

 restricted to a portion. The peculiar structure of the operculum, 

 to which those of Aulopoma and Pomatias only exhibit a faint 

 resemblance in their hollow construction, has hitherto escaped 

 observation. 



Genus Opisthoporus y nobis. 



Operculum calcareum, circulare, crassiusculum, multispiratum, du- 

 plex, utrinque concaviusculum ; disco interne, epidermide cornea 

 lubrica vestito, externo calcareo, scabro ; duobus lamina spirali, 

 erecta recurva interposita, junctis ; anfractuum interstitiis interne 

 vacuum prsebentibus ; margine circumdante eoncavo. 



Testa depressa, orbicularis, late umbilicata, sutura pone aperturam 

 tubulo exserto, pervio, munita. Peristoma duplex, externum ex- 

 pansum, superne antice subfornicato-alatum, internum superne in- 

 terdum emarginatum breviterque incisum. 



Sp. 1. Opisthoporus biciliatus, Mousson. 



Syn. Pterocyclos biciliatus, Mouss. Java Moll. p. 49. tab. 20. 

 f. 9 (1849). 

 Cyclotus Taylorianus, Pfr. 1851, Zeitschr. p. 7. 

 Cyclostoma (Fterocyclos) Charbonnieri, Reel. Journ. 

 Conch. 1851, vol. ii. p. 214. t. 5. f. 12, 13. 



Sp. 2. Opisthoporus rostellatus, Pfr. 



Syn. Cyclotus (Cyclostoma), Pfr. Zeitschr. 1851, p. 8, and 

 Mon. p. 40. 



Sp. 3. Opisthoporus spiracellum, Ad. and Reeve. 



Syn. Cyclostoma spiracellum,, ditto, Voy. Samarang, p. 56. 

 t. 14. f. 1. 



Sp. 4. Opisthoporus tubuliferuSy Pfr. 



Syn. Cyclotus tubuliferus, ditto, Mai. Bliitt. vol. i. p. 31. 



The circumstance of the tube in the suture of the last-quoted 



