COLUMELLAR MUSCLE AND OPERCULUM. 15 



homologue, of the byssus of the lamellibranchs ; and is certainly 

 not homologous with either of the valves of the shell of the 

 latter, which are pallial structures." * 



The following interesting 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 

 probable restoration when entirely destroyed. 



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

 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. 

 That such an occurrence would most probably be rare, is easily 

 explained from its situation, as the operculum is protected by 

 the last whorl of the spire of the shell when the animal is 

 expanded, and by the mouth when it is contracted into the 

 cavity of the shell. 



u I have lately met with a very distinct example in a specimen 

 of Fusus in the British Museum collection. In this specimen 

 the apical half of the operculum has been broken off (pi. 2, fig. 

 12 a), and the lost part has been renewed by an irregular round- 

 ish process, nearly of the size of the lost part, not quite as thick 

 as the original portion, and formed of rather irregular horny 

 plates ; the smaller or first-formed portion being in the centre of 

 the broken line, so that the restored part bears some similarity 

 to the annular operculum of a Paludina. This restoration is 

 exactly like that which would have taken place in a shell under 

 similar circumstances, and is a further proof of the truth of the 

 theory which I have long advocated, that the operculum is a 

 rudimentary valve, and is homologous to the second valve of the 

 bivalve mollusks. 



" In examining two specimens of Pleurotoma babylonica, pre- 

 served in spirits, with the opercula attached, I was much sur- 

 prised to observe that the opercula of the two specimens were 

 exceedingly different in structure and belonged to two distinct 

 modifications of that valve, one (pi. 2, fig. 12 b) being sub- 

 annular, with the nucleus apical, like the other species of the 

 genus, and the other (pi. 2, fig. 12 c) annular, with the nucleus 



Huxley, Anat. of Invert., 487, 1877. 



