116 THE NAUTILUS. 



The operculum of Leptopoma is essentially that of many species of 

 Cyclophorus (compare Leptopoma sericatum, Pfr. with C. fulguratus , 

 Pfr. and C. borneense, Mtf.). It is membranous, corneous, arcti- 

 spiral suppressedly striate, on the laminae, concave, with a polished 

 inner surface. The fimbriated shaggy exfoliated surfaces of many 

 opercula of the larger Cyclophorus (see C. arthritis, C. nilagirianus, 

 C. oculus-capri , C. tuba, C. validus, etc.) are age characters, and 

 seem also to have some relation to the moist habitats of the animals. 

 This type of operculum is repeated in Megalomastoma, though the 

 shell in the latter case is pronouncedly and divergently different (see 

 M. ventricosum, M. vemiculosum.') 



Cataulus is in its operculate character identical with the fore- 

 going. Although from the size of the apertures the opercula are 

 small, they are in construction, substance, expression, and micro- 

 scopic features, inseparable from the opercula of Cyclophorus, Mega- 

 lomastoma, and Leptopoma. The flatness usually observable in the 

 opercula of the last two genera is shown in Cyclophorus in small 

 shells, the concavity of the opercula of the latter genus being the 

 result of the natural curvature produced by the protruding fibres of 

 the larger foot in larger species. 



The operculum of Pupina is unmistakably indicated in structure, 

 nature, and physical appearance with all the foregoing, its tenuity alone 

 and flatness offering only the most evanescent and unreal discrimina- 

 tion from the other genera. Pupinella, Registoma, Callia, are quite 

 inseparable in their operculate character. Aulopoma is a shell very 

 near in external characters to Pterocyclos, Opisthoporus, Spiraculum, 

 Rhiostoma, omitting the lip expansion and spiracle, but the oper- 

 culum is sharply contrasted. The operculum in Atilopoma is corneous, 

 consisting of a flat spiral made up of tubular whorls coarsely striate 

 and embracing, cap-like, the margins of tbe shell aperture. If t lie 

 distinction of this genus is based upon the operculum it rather forcibly 

 separates a shell from its natural generic position. 



In structure, at least, the sub-orbicular operculum of Otopoma 

 naticoides Rec, with its cycloidal development with the tangential 

 strise leaving the whorls and extending outward, but forming flexu- 

 ous lines on the limits of the last whorl, is repeated in the authentic 

 opercula of Cyclophorus metabletus Crosse & Fischer. 



The instances of exact repetition in the opercula of Cyclotus and 

 Cyclophorus ase numerous. The operculum of Cyclotus translucidus 



