THE NAUTILUS. 1 15 



The opercula of the small shells Alycaeus and Diplommatina are not 

 often found in shells in collections. Where I could examine them 

 under a one-inch objective they certainly offer no determinative dif- 

 ferences from the opercula of Cyclophorus, and like the latter are 

 horny (corneous). Compare for instance the opercula of Alycaeus 

 rathouisiana, Hende, Cyclophorus parapsis, Benson. 



The genus Pterocyclos is certainly closely allied to genera Opis- 

 thoporus, Spiraculum, Rhiostoma, but the opercula of these four 

 genera exhibit two contrasted forms. Taking Pterocyclos angulif- 

 erus Soul, as an example of one form, the operculum forms a calcare- 

 ous button, concave upon the outer side, interiorly filmed by a horny 

 scale covering its entire width, grooved at the sides, and showing on 

 its exposed surface closely wound narrow whorls, throughout ob- 

 liquely striate. This configuration and structure is quite as naturally 

 and truly referable to the opercula of some species of Cyclotus as G. 

 auriculata, Kob., etc. 



This form of operculum is found in some species of Opisthoporus 

 (vide biciliatus, Mouss., birostra, Pfr.) and the distinction made by 

 II. and A. Adams between the opercula of Opisthoporus and Pterocy- 

 clas is certainly misleading if universally applied. Another more 

 common type of operculum, seen in Spiraculum, Rhiostoma, some 

 species of Opisthoporus and Pterocyclos is a spiral lamina, concave 

 interiorly, more or less deep with the free edges of the laminae erect 

 or explanate and horizontal, arising screw-wise steeply around a 

 solid nucleus {Rhiostoma) or more spreading with corneous intercal- 

 ation {Spiraculum). The morphology of the shells themselves sep- 

 arates these genera, though it is probable the distinctions should form 

 subgenera, but the opercula present two forms, which while partially 

 restricted are surely not enough so to offer any basis for strict 

 classification. 



But the confusion, so far as opercula are considered, does not end 

 here. The opercula of the second type of Pterocyclos, Opisthoporus, 

 Spiraculum, Rhiostoma, in substance and structure, i- practically re- 

 peated in those of Qhoanopoma. Both groups show the exsert, er< 

 or spreading laminae, the spiral curvature, the union of calcareous 

 and corneous texture, though the inner surface of the operculum in 

 Choanopoma is usually fiat, and in some instances as C. pulchrum, 

 Gray, the coarse expansive character of the whorls contrasts with 

 the analogous feature in Pterocyclas, etc. 



