Dr. J. E. Gray on the Operculum of the Genus Diplommatina. 9 



III. — On the Operculum of the Genus Diplommatina. 

 By Dr. J. E. Gray, F.R.S., V.P.Z.S. &c. 



To the Editors of the Annals of Natural History. 



Gentlemen, 



If Mr. Benson, before writing his observations on this genus 

 which appeared in the preceding Number of the ' Annals ' 

 (vol. xi. p. 433), had taken the trouble to come and examine the 

 specimens of Diplommatina in the British Museum on which Dr. 

 Pfeiflfer and I had founded our observations, he wovdd have 

 found that there was not the slightest ground for any of the 

 arguments which he has used to induce naturalists to believe 

 that the opercula described as belonging to the genus could have 

 been accidentally placed in the shell, and thus excuse the imper- 

 fection which occurs in his and Capt. Hutton's description of the 

 animal ; and further, he could never have made the extraordinary 

 suggestion that the opercula belonging to Diplommatina " were 

 adventitious," and might be " assignable to the young Alycceus 

 strangulatusj" for the opercula of the two genera are most un- 

 like in structure and colour, and that of the latter genus is at 

 least five times as large as the largest species of the former. 



The opercula of the three species of Diplommatina costulata 

 in the Museum are each attached to the dry remains of the ani- 

 mal ; two of the animals are still in the shell, and the third was 

 extracted from the shell for the purpose of more accurate exa- 

 mination. 



It is easy to understand, when we consider the minuteness of 

 the operculum, its small size compared vdXh that of the mouth 

 of the shell, and its transparency, how it may be overlooked, 

 especially when it is sought for in the curious manner mentioned 

 by Capt. Hutton. I have a strong suspicion that if Mr. Benson's 

 specimens were more cai'efully examined, the operculum would be 

 discovered, unless the animal has been eaten out of the shell 

 by insects. 



The operculum of Acme fusca, so common in many parts of 

 England, which is of about the same size but darker, was over- 

 looked by many malacologists, and has been denied after it was 

 described by others, as is the case with that of Diplommatina. 



I may observe, merely to try to clear away certain theories 

 which continue to cling about malacology', that I cannot consider 

 " the existence of the tooth-like plait on the columella " of any 

 force as ''militating against the theory of an operculum " in this 

 genus, for we now well know that Pijramidclla, Odostomia and 



