130 SACfAKTIADiE. 



point of union. In a third "beautiful specimen, sent me 

 alive, I found, after death, the membrane showing dis- 

 tinct concentric lines of growth. And these took exactlv 

 the form of the outer edges of the two lobes, meeting 

 in the centre, where there was a representative suture. 

 The growth-line being curved, there was a delta at 

 the end of the suture ; and this was filled with a much 

 thinner film of membrane, showing that it was the last 

 made. 



Mr. Walter Gregor, of Macduff, has sent me a large 

 specimen which had in youth chosen a shell of Natica 

 sordida for its support. The shell is in no direction more 

 than one-third of an inch in diameter, but the adventitious 

 body-whorl of membrane measures (along its curve) two 

 inches and three quarters ! 



From these and other observations of my own, as well 

 as from information supplied by Mr. Robertson, it appears 

 to me manifest that the membrane is a provision for the 

 support of the growing Adamsia, when it has selected 

 small or broken shells. 



Experiments, which I have detailed at length else- 

 where,* have satisfied me that the membrane is produced 

 by the Adamsia ; that it is an epidermic slough ; and that 

 it is composed mainly of chitine, having no calcareous 

 el -incut. It cannot, therefore, in any respect, be regarded 

 as a corallum. 



The membrane is not invariably present. In specimens 

 dredged in the Frith of Clyde, small or broken shells 

 appear to be usually chosen ; and these are enlarged, as I 

 have stated above. In Weymouth Bay, however, where 

 the species was common when I was there in 1853, the 

 shell most commonly selected being the Great Whelk, the 



* Annals and Mag. of Nat. Hist, for Aug. 1858. 



