130 
SAGARTIAD^. 
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 exactly 
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, 
j\Ir, AV alter 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 
element. 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 AVey mouth Bay, however, where 
the species was common when I was there in 1853, the 
shell most commonly selected being the Great AA'helk, the 
* Annals and Mag. of Nat. Hist, for Aug. 1858. 
