containing a moderate amount of pebble or coarse shell debris, which is 

 required for larval settlement. It prefers bare, rather than grass-covered 

 bottoms. This species is a moderately rapid burrower. The foot emerges 

 opposite the hinge and flows slowly into the substratum during a prolonged 

 probing period. Five or 6 burrowing sequences are normally required to erect 

 the shell. The erect probing orientation is with dorsal area just posterior 

 to the hinge horizontal. Angle of rotation is about 25°. Direction of 

 movement into the sediment is slightly forward (about 25°) from vertical. 

 Life position varies with substratum type. Animals in clean sand live more 

 deeply buried (average depth about 2 cm) than those living in muddy sediments 

 (average depth about 1 cm in 30 percent mud) . Individuals living in clean 

 sand sometimes strain water through the overlying sediment layer. Sand- 

 dwellers also eject pseudofeces less often and grow more rapidly than mud- 

 dwellers. Animals in compact sand normally live with the long axis of the 

 shell oriented approximately 45° from vertical, those in muddy substrata tend 

 to live with long axis nearly vertical. In intertidal sand, small animals 

 commonly live more deeply buried than large animals, apparently for 

 protection against disinterrment, to which they are more vulnerable. - W.J.B. 



1720 



Stanley, Steven M. 1972. 



Functional morphology and evolution of bysally attached bivalve mollusks. 

 J. Paleontol. 46(2): 165-212. 



Evidence is strong in favor of the hypothesis (attributed to C. M. Yonge) 

 that adult byssal attachment has arisen in the Bivalvia through neoteny. 

 Stabilizing adaptations of Mercenaries mercenaria change during growth. A 

 byssus is used for juvenile fixation until size confers greater stability 

 by permitting deeper burial and by increasing ratio of weight to shell 

 surface area. Four stages in development of M. mercenaria are illustrated 

 in figure 7: byssal plantigrade stage; first-year clam with sharp, concentric 

 costae for physical stabilization; young clam with relatively long siphons 

 for protection in a deep life position; and large clam with short siphons 

 in a shallow life position. If positions of life stages and lengths of 

 siphons are drawn correctly, which must be presumed because a 5 cm scale 

 is included, the relatively deep young stage is farther below the surface 

 of the substrate than the large clam, although their centers of gravity 

 appear to be about equally deep. The deep stage appears to be about 4 cm 

 long and the hinge is about 4 cm below the surface. The large clam is 

 about 9 cm long and the upper edge of the shell about 1 1/2 cm below the 

 surface of the substrate. As shown, ratios of siphon length to shell 

 length are about 1:1 and 1:5. - J.L.M. 



1721 



Stanley, Steven M. 1975. 



Why clams have the shape they have: an experimental analysis of burrowing. 

 Paleobiology 1(1): 48-58. 



Analysis of movies has shown that each rocking motion of a morphologically 

 typical clam, Mercenaria mercenaria, involves typically rotational movement, 

 with no translational component. The clam is able to burrow by "walking" 

 its way downward only because the axis of backward rotation lies to the 

 anterior of the axis of forward rotation. The blunt anterior serves to 

 shift the axis of backward rotation anteriorly, thus aiding in downward 

 progress. - modified author's abstract - J.L.M. 



1722 



Stasek, Charles R. 1963. 



Orientation and form in the bivalved mollusca. J. Morphol. 112(3): 195-214. 



Largely because early conchologists ignored the body, and developed a system 

 of orientational terms based on features of the valves, the dorsal, ventral, 

 anterior, and posterior regions of the shell do not necessarily lie adjacent 

 to similarly named regions of the body. However, it is recommended that the 

 directional terminology be retained in Venus. - J.L.M. 



476 



