576 LILLIE. 



larva undergoes a longitudinal invagination to form the mantle, 

 they form the bottom of the invagination as far forward as 

 the mouth and become the anlage of the foot and pedal 

 structures. The lateral pits are derived from the same source. 



Thread-gla nd. 



The larval byssus gland, for which I will adopt the better 

 term, "thread-gland," proposed by Schierholz,^ is a unicellular 

 structure, which opens between five large cells placed just 

 ventral to the anterior end of the shell-gland. Its duct is, 

 therefore, intracellular. The cell from which it is derived 

 is ectodermic, and in earlier stages lies in the centre of the 

 five large cells just mentioned. 



The complete paper will contain an account of the further 

 development up to the glochidium stage. 



University of Chicago, 

 Feb. 3, 1893. 



1 Schierholz, Ueber Entw. der Unioniden. Denkschr. der Math.-Naturwiss. 

 Klasse Akad. Wien, LV. i{ 



