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 Neritaglaucina, in which the embryo shells are retained 

 and preserved, vmtil. they have arrived at sufficient growth to 

 obtain nourishment for themselves, and to support an indepen- 

 dent existence. 



XV. Dt 



