THE NAUTILUS. 



117 



Sowb. is certainly reproduced in those of Cyclostorna xanthocheilus, 

 C. barclayanus, G. campanulatus. The Hat thin spiral, almost equal 

 with the slightly overlapping ascent outward to the last whorl, striate 

 on its exterior edges, is closely repeated in each. 



In Adamsiella gray ana Pfr., A. variabile Ad., there is a spiral oper- 

 culum with exsert laminae, somewhat multifolded or lamellose on 

 edges, but structurally distinctly like Choanopoma and Opisthoporus. 



The operculum of Tudora and Cistula with their rapidly increasing 

 whoris, the last expansive and filling one-third of the aperture, nuc- 

 leus subcentral, are practically identical. The thin substance of the 

 operculum of C/iondropoma gives it some essential differentiation, but 

 in the theory of its form it is exactly like Tudora and Cistula. The 

 outer surface of the operculum of Tudora is frequently ( T. shepardi- 

 anus Ad., T. pupoides Morelet) fibrous-rayed, whereas in megacheilus 

 its surface is smooth, or obsoletely striate which form is closely imi- 

 tated by the operculum of Ckondropoma. 



The following table exhibits the substance of the operculum in the 

 genera of round-mouthed shells, as observed in the collection of the 

 American Museum of Natural History, and as given by Pfeiffer. 

 The two columns afford slight differences, but such differences might 

 readily be referred to individual variations in the shells examined by 

 two observers, using different groups of specimens. 



SUBSTANCE OF THE OPERCDLUM. 



Observed in Collection. 



Testaceous to subtestaceous. Sub- 

 testaceous in C. crassus, Ad. C. 

 triliratum Pfr., C. rugatus Guppy, 

 C. suturale Swb. 



Corneous. 



Corneous. 



Testaceous to Sub-testaceous in P. 

 rupestris Bens. 



Not seen. 



Corneous. 



Corneous. 



Corneous. Membranous. 



Corneous. 



Corneous. 



Corneous. 



Corneous. 



Cyclotits. 



Alycieus. 



Diplommatina. 



Pterocyclos. 



Craspedopoma. 



Aulopoma. 



Cyclophorus. 



Leptopoma. 



Megalomastoma. 



Cataulus. 



Pupinella. 



Pupina. 



Pfeiffer. 

 Testaceous. 



Sub-testaceous. 

 Sub-testaceous. 

 Sub-cartilaginous. 



Corneous. 



Corneous. 



Corneous. 



Membranous. 



Corneous. 



Corneous. 



Corneous. 



Membranous. 



