280 LIFE BY THE SEASHORE. 



and the pallial sinus deep. Of the three genera named, 

 the species of Mactra are most commonly found in the 

 living condition, and are also very common as shells on the 

 beach at all seasons. Two of the species, M. solida and 

 M. subtruncata, are very much alike, and not easy to dis- 

 tinguish from a description merely ; while the third species, 

 M. stultoruwi, or Fool's Mactra, is readily recognised, and 

 cannot be confused with any other shell. In all cases the 

 shells are triangular, and are characterised by the almost 

 smooth surface and the nature of the teeth. Of these there 

 are two thin cardinals in the right valve, and two similar 

 but united cardinals in the left; the laterals are large and 

 laminar, there being two on each side in the right valve 

 and one on each side in the left. In Mactra solida the 

 shell is solid, opaque, and perfectly triangular, the sides 

 being equal ; the surface is marked by concentric striae and 

 is yellowish white in colour, often stained by substances 

 derived from the sand. It is not easy to point out dis- 

 tinguishing differences from M. suUruncata, but the latter 

 is smaller, more convex, and seems to be hollowed out at 

 either side of the beaks, so that 

 these become more prominent. In 

 M. stultorum the shell has the same 

 shape as in M. solida, but is thin, 

 delicate, glossy, and almost smooth. 

 The colour is a pale brownish tint, 

 variegated by longitudinal rays of 

 reddish brown. The shell is very 

 familiar, and is represented in all 



FIG. 80. Mactra stultorum. ,-, ,, ,. * , ., , ,, 



the collections of children; the 



other species, on the other hand, being thick and clumsy, 

 are often neglected. 



As members of the same sub-order we may mention two 

 other forms, abundant as shells, but not commonly found in 

 the fresh condition. One of these is Donax vittatus, the 

 purple toothed-shell, an active little form which lives in 

 sand near low-tide mark, whose shells are greatly prized 

 by children both for their beauty and colour varieties. The 

 living animal is both interesting and beautiful, the foot 

 being large in proportion to the body, the mantle delicately 

 fringed, and the siphons, which are quite separate, marked 



