14 COLUMELLAR MUSCLE AND OPERCULUM 



2. The operculum is generally horny and formed of a sub- 

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

 like the Bulla, Aplysia, certain Uniones, etc., are entirely or 

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

 when inhabiting granitic regions, are equally of epidermal sub- 

 stance : on the other hand many opercula are thickened inter- 

 nally with a calcareous deposit. 



Dr. Gray 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 at by Dr. Gray is that the normal or 

 typical form of mollusks is that protected by two valves or 

 shells : indeed, some nudibranchiate gasteropod mollusks which 

 have no shell in their adult state (Doridse), have their newly 

 hatched young covered with two shelly valves which afterwards 

 fall off.* 



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, with- 

 out opercula. I have already alluded to the supposed opercular 

 bodies found with the fossil Ammonites : f it may be added that 

 they occur as well in some of the Heteropods and Pteropods. 



In 1847, Loven proposed to consider the operculum as analo- 

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

 genera of Gasteropods provided with an operculum, secrete a 

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

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

 Lo ven's views : 



" On the hiemal aspect of the posterior portion of the foot, 

 a chitinous or shelly plate, termed the operculum, may be de- 

 veloped. This operculum appears to be the analogue, if not the 



* Ann. Mag. Nat. Hist., 2 ser., v, 476, 1850. 

 f MAN. OF CONCH., vol. i, Cephalopoda, 267. 



