OBSERVATIONS ON LIVING SOLENOMVA. 



267 



the thick stocky foot is seen occupying the whole space (Fig. i). 

 At the point of junction of the two sides of the pedal opening 

 there are crowded together muscle fibers, producing a silvery 

 appearance, and at this point muscle fibers that run parallel to 

 the borders of the pedal opening which Drew described as in 

 the nature of sphincter muscles cross to opposite sides and 

 continue their course posteriorly. May not the origin of the 

 peculiar sharply defined cruciform muscles in Solecurtus, Tagelus, 

 Macoma and Tellina be traced to these separate crossing muscle 

 fibers in some primitive lamellibranch like Solenomya? 



FIG. 2. FIG. 3. FIG. 4. 



FIG. 2. Pedal opening showing white area, W. 



FIG. 3. Side view of animal. 



FIG. 4. Papilla on margin of pedal opening. 



The edge of the pedal opening is at times reflected and at its 

 junction in the median line a few blunt papillae or tubercles appear 

 along the edges to nearly half the length of the opening. These 

 are so aligned that when the two edges come together in closing 

 the tubercles interlock. In Fig. 2 the tubercles are seen with 

 the pedal opening nearly closed. They vary in size, are con- 

 tractile and stand out at a sharp angle from the border so that 

 they are distinctly seen when the animal is viewed from the 

 side (Fig. 3). Until nearly adult these tubercles are translucent, 

 but in the oldest specimens they become opaque white, re- 

 sembling white kid, and appear hard and horny until touched 

 when they partly retract. They are covered with a transparent 

 epithelium (Fig. 4). This peculiar whitening not only covers 

 the tubercles but whitens the mantle border from which they 

 spring. A blotch of white is also seen between the tubercles and 

 the keen edge of the periosteum, which at this point is also 



