494 



proxima Say. All are found on very soft, muddy bottoms, in shallow 

 coves, along the coast of Maine, but they are never found associated 

 in very great numbers. N. delphinodonta is especially abundant near 

 shore, in shallow water, where the soft mud is filled with partially 

 decayed organic matter, and where the tidal currents are never very 

 strong. Y. limatula is also found in shallow water, but is generally 

 more abundant somewhat further from the shore, in soft mud in which 

 organic matter has undergone more thorough decomposition. Both of 

 these forms were found in great abundance. Young specimens of both 

 species were also dredged with special appliances. In the case of 

 N. delphinodonta the youngest specimens thus collected corresponded 

 in development to the oldest specimens reared from eggs, while many 

 specimens of Y. limatula but little over 0,5 mm in length, were obtained. 

 But few specimens of N. proxima were obtained. Those that were 

 collected were found on soft sandy-mud bottom, where the water was 

 several fathoms deep. 



Mantle. 



The margins of the mantle of both species of Nucula are smooth 

 and free along their whole length. 



The mantle of Y. limatula is provided with well developed siphons, 

 with a pair of rounded projections, probably tactile organs, opposite 

 the ends of strips that radiate from the breaks to the antero-ventral 

 margins of the shell, with a pair of flattened expansions, fringed with 

 tentacles, also very sensitive to touch, opposite similar strips that 

 radiate to the postero-ventral margins of the shell, with a series of 

 tentacles along the postero-ventral margins, and with a single unpaired 

 tentacle near the base of the siphons. 



The siphons, Fig. 1 es and is, are united throughout their whole 

 length, and may be extended beyond the shell to a distance consider- 

 ably greater than the length of the shell itself. In development, the 

 exhalent siphon is formed first, apparently by the union of the mar- 

 gins of the mantle, followed by growth into a tube, and the with- 

 drawal of the base of the tube into the mantle-chamber. A ridge on 

 the inside of each mantle-lobe, connects the base of the siphon with 

 the margin of the mantle. This ridge seems to indicate the margin 

 of the mantle, that has been drawn in with the receding siphon. The 

 inhalent siphon is formed ventral to the base of the exhalent siphon, 

 by the fusion of the ridges just described, accompanied by a dorsal 

 arching of the ventral wall of the exhalent siphon, which now forms 

 the dorsal wall of the inhalent siphon as well. The line of fusion on 



