5. 



mantle lobes. In withdrawing the tube its dorsal surface, cor- 

 responding to tlie dorsal surface of the united mantle lobes, is 

 drawn, in, forming a complete septum. A ridge on each mantle lobe 

 indicates ^j.ere the point of union of the mantle margins has been 

 drawn along its inner surface. 



The ventral surface of the base of this siphon arches 

 dorsally, figure 12, the ridges on the mantle lobes near its base 

 thicken and finally fuse. Thus a second tube is formed lying 

 ventral to the extialent siphon, figure 13. The wall separating 

 the two siphons reinains arcZaed upward for some t^ime, but ffabse- 

 quent growth straiglitens it. Even in the adult the line of 

 fusion along tlie ventral side of the inhalent siphon remains dis- 

 tinct, figure 14, and offers little resistance to splitting. Al- 

 thoufTh the inlialent siphon is formed between the mantle lobes, at 

 the base of the exhalep.t siphon, it may morphologically be con- 

 sidered marginal in formation, as it is formed by thickenings of, 

 and growth from, ridges that seem to have been carried back from 

 the margin. The adult siphons are united along their Tfoole 

 length, figures 1 and 3, and may be extended beyond the shell to 

 a distance considerably exceeding the shell's length. Normally 

 the inhalent siphon is shorter, broader, and has thicker walls 



