1872.] F. Stoliczka — On the Cyclostomacea of 'Fencing. 265 



Genus. OPISTHOPOKUS, Bens. 



There is no apparent distinction between the shells of Opisthoporus and 

 those of Spiraculum. A cursory examination of the animals of a few species 

 also shewed, that no essential distinction exists in the general anatomical 

 structure, and hut a very slight one in the dentition. 



The only difference, which as far as known is a constant one, consists 

 in the structure of the operculum. In the former genus this is discoid, 

 horny on the inner side, calcareous on the outer, and composed of spiral 

 laminse entirely covering a tube. In Spiraculum the upper spiral layer is 

 also generally calcareous, and more or less elevated, but the spiral canal 

 is always open, not forming a closed tube. The former structure of the 

 operculum is peculiar to Cyclotus, the latter to Fterocyclus and some species 

 of Cyclophorus. 



Opisthopoetjs Penang-ensis, n. sp. PI. X, Fig. 7. 



O. testa sub-discoidea, apice paulum exserta, latiuscule umbilicata, corneo 

 solidula ; anfractibus 4"5 ad 5, teretibus, sutura profunda junctis, epidermide 

 brunnea vel nigrescente, transversim confertissime striata, in ultimo an- 

 fractu ad peripheriam superam et infer am breviter ciliata, indutis, sub epi- 

 dermidem alhescentibus atque strigis transversis, brunneis vel fuscis, paulo 

 wndulatis, aut plus minusve acute angulatis, notatis ; apice sub-mammillato, 

 nigrescente vel pallido ; umbilico modico, fere dimidmm latitudinis anfractus 

 penultimi exponente ; ultimo anfractu ad aperturam valde descendente, sed 

 haud dissoluto, ad suturam tubulo brevi tenuiqtie, scepissime retrorsum curvato, 

 rare fere verticali, rarissimeque antice versus directo, in speciminibus adultis 

 circa 1*5 ad 2 mm. a margine aperturali distante, instructo ; apertura cir- 

 culari, ampla, obliqua, peristomate in junioribus simplici, in adultis breviter 

 bilabiato, margine labii interni paululum incrassato, scepe rubescente tincto, 

 haud distincter discreto, externo expansiusculo ; ambobus supra pauluni pro- 

 ductis atque prope suturam modice insinuatis. Operculum discoideum, 

 interne vix, externe distincte, concavum et album, multispiratum, medio 

 corneo -testaceum, laminis duabus separates ad peripheriam acutissimis. 

 Diam. may. 11*5 ; diam. min. 9"5 ; altit. testes 6'6 ; diam. apert. int. 4, 

 externa cum perist. 4*7 m.m. 



This species is evidently closely allied to E. v. Martens' O. Sumatra- 

 ntts* which is of nearly exactly the same size, but its whorls are decidedly 

 thinner and on the upper side more flattened, the ixpper apertural margins are 

 considerably more produced, the plain of the aperture being, therefore, more 

 oblique to the axis, while the sutural tube appears to be more distant from 

 the margin. The direction of the tube was observed in fom 1 specimens of 

 O. Sumatranus to be nearly vertical, while in about eighty specimens of the 

 * Preuss. Exped. nach Ost-Asien., Zool. Theil, vol. II, p. 112. 



