MORSE: OBSERVATIONS ON LAMELLIBRANCHS. 
189 
in a sharp spiral of two or three whorls like the shell of Lymnea 
gracilis. The inner palpus was more active than tlu; outer one. 
The young shell, less than a millimeter in length, is one of the 
most common objects sifted from beach sand. The protoconch 
is circular and tumid (Fig. 41, 6). 
MACTRA LATERALIS Say. 
Fig. 42. Length, 25 mm. 
The foot is large, fat and pointed; the syphons are closely 
united and when extended exceed the length of the shell. They 
are slender and have a slight upward curve. The color is very 
light brown. The papillae fringing the opening of the branchial 
syphon are twenty-four in number, long and short ones accurately 
alternating. A row of brown dots marks the space between the 
papillae, and a row of larger dots encircles the syphons ju-st below. 
The papillae are transparent with white tips with a middle light 
Fig. 42. — Mactra lateralis Say. 
brown line as in M. solidissima. The valve is long and tapering, 
extending beyond the tips of the papillae. A ring of minute 
papillae is seen on a line with the ring of dots on the branchial 
syphon. The animal is very active in its movements, the foot 
moving with great vigor and twirling the shell about as in Mactra 
solidissima. This species must have been very common along 
the coast in past times, as it is turned up by the hundreds when- 
ever a ditch is dug through the peat and mud. 
