380 Mr. W. H. Benson on the genus Diplommatina. 



both, the opercula being similarly remote, and retained fully 

 two whorls from the opening of the shells. I lose as little time 

 as possible in acknowledging the correctness of Dr. Gray^s 

 reference of the genus to the Cf/clostomacea, and in making 

 known the discovery that my former opinion on the subject is 

 untenable. 



The expectation of finding the operculum at or near the aper- 

 ture, induced by Dr. Gray's statement in a note dated in February 

 1853, that the operculum was '^ the usual external one, the size 

 of the mouth of the shell," had caused me to stop short of the 

 sacrifice of specimens in which no trace of the operculum could 

 be perceived from the aperture ; but the statement in his pub- 

 lished note regarding its extreme minuteness created a suspicion 

 that it might be found in a remote part of the shell, and this 

 expectation was so fully borne out by experiment, that the 

 unusual retraction of the accessory valve must be regarded as a 

 generic character in Diplommatina ; and if Dr. Gray's examples 

 occurred within view from the aperture of unbroken specimens, 

 we are authorized to conclude that the animals must have been 

 suddenly killed, and prevented from retiring to their usual re- 

 condite position. The smallness of the operculum also explains 

 why the tooth on the internal part of the columella affords no 

 obstacle to the passage of the valve to the remoter whorls, and 

 why no modification of its circular form is necessary for the 

 execution of that movement, in spite of the apparent obstruction 

 presented. 



Dr. Gray correctly reminds us that " the operculum of Acme 

 fusca was overlooked by many malacologists, and has been denied 

 after it was described by others, as is the case with that of Di- 

 plommatina /' for, in the list of those who contested its existence 

 in Acme, we find the honoured name of De Ferussac, and at as 

 late a date as that of the publication of Gray's edition of ' Tur- 

 ton's Manual *,' the now patent fact was opposed, and the shell 

 was classed, like Diplommatina, with Carychium, instead of with 

 its real ally, Cyclostoma elegans ; so that the supporters of the 

 Carychiadous affinity of Diplommatina have erred in good com- 

 pany — 



" veniam damns petimusque vicissim.'* 



The fact seems to be, that in these minute shells the exceeding 

 delicacy of the transparent operculum causes its concealment, 

 during life, by the mucus of the animal, and it is not until this 

 has dried up, and become more or less separated from the hard 

 covering, that the latter becomes visible: in A. fusca it remains 

 in the mouth of the shell, but being so much smaller than the 



♦ Edition of 1840. 



