commonly called Flustra arenosa. 321 
be precipitated, and will leave a membranaceous covering on 
both sides of the shells, and make the small shells contained in 
them very conspicuous. 
On comparing what has been said of this substance, with the 
character of the genus Flustra, or of any other genus belonging to 
the order Vermes Zoophyta, it will be evident that it cannot be 
referred to that genus; but is rather to be considered as the 
matrix of Nerita glaucina, in which the embryo shells are retained 
and preserved, until they have arrived at sufficient growth to 
obtain nourishment for themselves, and to support an indepen- 
dent existence. 
XV. De- 
