Mr. W. H. Benson on the Genus Opisthoporus. 15 



species being directed forwards will not render any change ne- 

 cessary in the name, the tube being still posterior with reference 

 to the aperture. The description of the mouth, tube, and oper- 

 culum of O. spiracellum, brought by the Samarang from an 

 island off the N.E. point of Borneo, permits no doubt of its place 

 being in the proposed genus. Pfeiffer has placed it, with a 

 mark of doubt, in Pterocyclos, while he surmises that it may be 

 the same species as the Singapore rostellatus. The inflated back, 

 and strangulation of the last whorl militate against this con- 

 clusion. The genus occupies a range from Singapore, through 

 Borneo, towards the Manilla group. 



The place which Opisthoporus holds will be found, as fixed for 

 the type by Pfeiffer, between CydotuSy to which it is linked by 

 the abnormal species C. variegatus, Swainson, and Pterocyclos ; 

 the aberrant form, Pt, hispidus, Pearson, forming the passage 

 to the typical species of the latter genus, and agreeing with 

 Opisthoporus in the aperture and sutural tube ; while it holds to 

 Pterocyclos by the operculum. Not having C. variegatus at 

 hand for comparison, I cannot say if the internal structure and 

 edge of the thickened operculum resemble that of Opisthoporus, 

 but, in the absence of the spiral elevated lamina at the edge of 

 the turns, it decidedly gives warning of a departure from the 

 received Cyclotoid type. If it should be found eventually to 

 exhibit the structure of the operculum of 0. biciliatus, inasmuch 

 as the suture is destitute of a tube, the name which I have im- 

 posed will be held inapplicable to all the species, and, in that 

 case, it may be desirable to use one of more general application, 

 when the term Coslopoma^, nobis, may be substituted. 



It may here be remarked that, on the strength of the tecti- 

 form canaliculate wing, Pfeiffer has admitted Cyclostoma breve, 

 Martyn, and C. planorbulum, Lamk. (genus Myxostoma, Trosch.) 

 into Pterocyclos, from which the difference of substance and the 

 plane laminar structure of the multispiral opercula appear wholly 

 to exclude them. Now as C. planorbulum of the ' Encyclopedic 

 Methodique ^ was clearly the typical species of Cyclotus, Guild- 

 ing, as made known by Swainson, it may be ultimately advisable 

 to separate this shell from Pterocyclos under Guil dingus name, 

 and to restore TroschePs name Aperostoma to the shells bear- 

 ing the form of operculum which Troschel had especially in view 

 in proposing that division. After C. planorbulum, Swainson 

 quoted C. variegatum as an additional species of the type ; and 

 it will depend upon the internal structure of its operculum, 

 whether, as is most probable, it should be associated with planar- 



* Coslopoma — koIXos, cavus, and ntofia, operculum. 



