60 TLYANASSA. 



N. TRIVITTATA, Say. PL 18, fig. 346. 



White or yellowish white, sometimes faintly marked by three 

 revolving light brown bands. Suture channeled, with usually a 

 bead-like row of small tubercles below. it, and separated from the 

 cancellated and tuberculated surface by a slight impressed line. 



Length, '7-*9 inch. 



Massachusetts to Florida. 



The animal is whitish, sparsely clotted with pale lilac ; foot 

 slightly bifid behind with two erect subulate processes. Very 

 active. Comes out of the sand towards low-water mark, in a 

 minute or two after the water passes over them. On most 

 specimens the bands on the shell are either obsolete or so faintly 

 marked as to escape hurried observation. 



s IlyanaSSa, Stiiup.-on. 



The generic characters proposed by Stimpson include an 

 operculum without serrated margin, arid the animal without 

 posterior bifurcation. Although the operculum is usually 

 crenated in Nasm, Mr. Marrat has enumerated a dozen species 

 in which it has been observed to have plain margins, or nearly 

 so ; and Dr. Von Martens states* that the European N. reticulatu- 

 is found in the mud-flats of the Venetian lagunes with the oper- 

 culum plain on one side and somewhat serrated on the other, 

 and that the end of the foot is but slightly notched in these 

 specimens instead of being deeply bifurcated. Under these cir- 

 cumstances, it becomes very doubtful whether the group 

 Tlyanassa ought to stand. I have concluded to retain it pro- 

 visionally as a subgenus, especially as it may include several 

 species conveniently separable from Tritia by having dark- 

 colored shells. 



N. OBSOLETA, Say. PI. 18, figs. 347-349. 



Chocolate-brown or olive, with occasionally a faint, lighter 

 colored central band ; deep chocolate within the aperture, with 

 a central white band. Length, '75-1 inch. 



Massachusetts to Florida. - 



The animal is variously mottled with slate color, the tentacula 

 are suddenly diminished above the eyes, and become bristle-like. 



* Zool. Record, ii. 244. 



