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



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

 appearance 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 Cijclostomacea rest. I do not hesitate, there- 

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

 posed in my correspondence with Dr. L. PfeifFei', 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 Ali/cceus and Pterocyclos, which Albers has 

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

 schrift flir Malahozoologie ' for 1852. Dr. Pfeiffer opposed that 

 view, and, while objecting to the separation of 0. Taylorianus 

 from Cijclutus, 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, nobis. 



Operculum calcareum, circularc, crassiusculum, multisph'atum, du- 

 plex, utrinque concaviusculum ; disco interiio, epidermide cornea 

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

 erecta recurva interposita, juuctis ; anfractuum interstitiis interne 

 vacuum prsebentibus ; margine circumdante concavo. 



Testa depressa, orbicularis, late umbilicata, sutura pone aperturam 

 tubuio cxserto, pervio, munita. Peristoma duplex, externum ex- 

 pansum, supen.e antice subforiiicato-alatum, internum superne in- 

 terdam emarginatum breviterque incisum. 



Sp. 1. Opisthoporus hiciliatus, IMousson. 



Syx. Pterocyclos hiciliatus, Mouss. Java Moll. p. 49. tab. 20. 

 f. 9 (1849). 

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

 Cyctostoma (Ftei-ocyclos) Charhomiieri, Reel. Journ. 

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



Sp. 2. Opisthoporus rostcllatus, Pfr. 



Syn. Cyclotus {Cyctostoma), Pfr. Zeitschr. 1851, p. 8, and 

 Mon. p. 40. 



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



Syn. Cyctostoma spiraceltum, ditto, Yov. Samarang, p. 56. 

 t. 14. f. 1. 



Sp. 4. Opisthoporus tuhuUferus, Pfr. 



Syn. Cyclotus tubutiferns, ditto, Mai. BlJitt. vol. i. p. 31. 



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



