4. 



(20) 



Tryon refers to this mark on the right valve, and 



thinks it is caused by the loop of the intestine that extends for- 

 ward, on the right side, very near the shell. That this view is 

 wrong is shown by there being similar markings on both valves. 



MANTLE. 

 Figures 1 and 3. 



The mantle lobes are free along their ventral borders, 

 and are ciliated in patches on their inner surfaces. They are 

 modified to form the siphons, the marginal tentacles^ ar» unpair- 

 ed, very rouch elongated tentacle situated near the base of the 

 siphons, a flattened expansion opposite the e>rbremity of each 

 posterior shell stripe, and a rounded projection opposite the ex- 

 tremity of each anterior shell stripe. These will be treated 

 in turn. There are also two pairs of apparently glandular patch- 

 es, one situated ventral to the anterior adductor muscle and the 

 other ventral to the base of the siphons. 



Siphons -- The oldest specimens reared from eggs have not begun 

 to develop siphons. The youngest specimen collected with a dredge 

 (about 1/2 m m. long) had already formed the exhalent siphon. 



This siphon, figure 11, seems to have been formed by 

 the union of the margins of the mantle lobes, followed by their 

 gro-vrbh into a tube and the withdrawal of the tube between the 



