114 THE NAUTILUS. 



III. Operculum entirely cartilaginous. 



Chondropoma. Operculum oval, subcartilaginous, plane, few whorls, rapidly 

 increasing, nucleus generally quite eccentric. 



IV. Operculum double interiorly concamerated. 



Pomatias. Operculum cartilaginous, few-whorled, made up of two laminae, 

 interiorly concamerated. 



V. Operculum corneous. 

 Genera, Realia, Omphalotropis, Bourciera. 

 Realia. Operculum thin, horny, few-whorled. 

 Omphalotropis. Operculum thin, horny, few-whorled. 



Bourciera. Operculum oval, somewhat solid, horny, with few rapidly in- 

 creasing whorls. 



Later authors have extended this list of genera, but Pfeiffer's 

 synopsis practically embraces the important and distinguishing gen- 

 era. The force assigned to the Operculum as separative of the genera 

 naturally appears exaggerated, when their characters are thus iso- 

 lated, but in view of the purpose of this paper to emphasize their 

 secondary, or in cases, entirely negligible weight, this isolation serves 

 the more explicit object of fixing attention solely upon the operculate 

 features. 



To begin with, in the genus Cyclotus, there exist differences in 

 the opercula of many species almost as great as that between the 

 opercula of recognized genera. In the species C. corrugatum, Swb. 

 the whorls of the laminae in the operculum are margined by erect 

 incurved slightly striate free fillets, the whole operculum presenting 

 the appearance of a watch-spring; whereas in typical examples of the 

 operculum of Cyclotus, the laminae are flat with edges strictly in 

 contact or slightly overlapping, the latter feature becoming extreme 

 in seminudum. There is here no essentially different principle of 

 construction involved but the formal contrast in appearance might as 

 safely be invoked to make another genus in the case of C. corru- 

 gatum (as it has been) so far as opercula offer signs of generic dis- 

 tinction. Troschel and H. and A. Adams have indeed placed this 

 shell, along with asperulus, cingulatus, crassus, etc., in all twenty- 

 five species, in the subgenus Aperostoma. 



The shelly substance of the operculum of Cyclotus is however a 

 quite constant feature. The shelly opercula are found in such small 

 shells as fodiens, hunanus, parvulus, minimus, where it might be ex- 

 pected that the calcareous secretions would be less complete. 



