76 



THE NAUTILUS. 



tubes enlarge as if about to burst, becoming semi-globular at the 

 ends ; then suddenly collapsing the tubes become narrow and opaque- 

 white in color. The tubes may perform this action independently. 



In the description of the tubes of Thracia distorta a behavior is 

 indicated precisely as seen in T. conradi. " Tubes globularly in- 

 flated at extremity, which increases whole length and then suddenly 

 collapses." 



The most marked peculiarity of T. conradi is the sharply defined 

 collar which surrounds the base of the siphonal tubes. This collar 



is a prolongation of the mantle, with an extension of the periostracum, 

 as seen in many other lamellibranchs, but in no instance have I met 

 with a description of any structure approaching the collar of T. 

 conradi. It flares like the c6rolla of a flower, and its edges are 

 reflected as shown in the figure here given. This prolongation of 

 the posterior border of the mantle is seen in other forms, but in no 

 case with the definition or freedom from the base of the siphonal 

 tubes as seen in this species. 



In Saxicava and Mya the prolongation of the mantle is closely 

 adherent to the tubes. My own observations show that in Anatina 



