THE OPERCULUM. 49 



surface, showing how the A'alve has rotated on the bodj?^ of the 

 animal as tlie operculum rotates on the foot of the gastropods. 



2. The operculum is generall}' hornj^ and formed of a substance 

 similar to the epidermis of shells ; but then some shells, like the 

 Bulla, Aplysia, certain Uniones, etc., are entirel}'' or almost 

 destitute of calcareous matter, and some of the helices, when 

 inhabiting granitic regions, are equallj^ of epidermal substance : 

 on the other hand many opercula are thickened internally with a 

 calcareous deposit. 



Dr. Gra}^ proceeds to show that in bivalve shells like Chama, 

 where one valve (the attached one) has a spiral apex, whilst the 

 other valve is a flattened spire, the position of the hinge with 

 reference to the spire must rotate slowly with growth, as in the 

 spiral operculum in its growth. The direction of the spire of a 

 spiral operculum is opposite that of the shell, showing another 

 analogy. 



The conclusion arrived a{ by Dr. Gray is that the normal or 

 typical form of molkisks is that protected by two valves or 

 shells. 



Dr. Gray has always maintained that the opercula are of great 

 value in the distinction of genera, and he does not fail to condemn 

 severely the practice of preserving shells in museums, or of figuring 

 and describing them in conchological works, without opercula. 

 I have already alluded to the supposed opercular bodies found 

 with the fossil Ammonites : it may be added that they occur as 

 well in some of the Heteropods and Pteropods. 



In 1841, Loven proposed to consider the operculum as analo- 

 gous to the byssus ; but Dr. Gray has pointed out that some 

 genera of gastropods provided with an operculum, secrete a 

 byssus also (Rissoa, Cerrithium, Littorina, etc.). However, Prof. 

 Huxley, one of the latest and best authorities, thus endorses 

 Loven's views, which are now generally adopted : 



" On the hsemal aspect of the posterior portion of the foot, a 

 chitinous or shellj^ plate, termed the operculum, may be developed. 

 This operculum appears to be the analogue, if not the homologue, 

 of the byssus of the lamellibranchs ; and is certainly not homo- 

 logous with either of the valves of the shell of the latter, which 

 are pallial structures." — Huxley, Anat. of Invert., 487, 187*1. 



The following note by Dr. Gray, will throw some light on this 

 very interesting subject : 



" On the reproduction of the lost part of an operculum, and its 

 jorobable restoration ivhen entirely destroyed. 



" It is to be expected that the operculum of a gastropodous 

 mollusk may be sometimes broken or injured, but I have never 

 hitherto been able to find any very distinct example of the kind, 

 so as to study how the repair of the lost part would be effected. 



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