MAEINE MOLLUSCS 209 



The shells are rather thin, closed at both ends, blunts and 

 rounded at the anterior end, but straight and more pointed at the 

 shorter posterior end ; and the margins of the valves are very 

 finely grooved in such a manner as to resemble the milling of a 

 coin. Each valve has two central hinge teeth, with one long 

 lateral tooth on each side ; and the ligament is external and 

 prominent. The lobes of the mantle are fringed ; the siphons 

 are separate and diverging, but shorter and thicker than in most 

 of the other Tellinidce, and the foot is comparatively large, 

 flattened, and pointed. 



The genus contains many species, the commonest being, 

 perhaps, D. anatinus, the colour of which is yellowish, banded 

 with brown, and marked by a number of radiating white lines. 

 This colour, however, is due entirely to the thin, shining epidermis 

 that completely covers the valves ; and if this is rubbed off the 

 shell itself will exhibit a pale pinkish tint. Another common 

 species (D. politus) may be recognised by the broad patch of white 

 running from the hinge to the margin, on the posterior side of the 

 middle of each valve. 



The family Mactridez contains some British shells popularly 

 known as Trough shells, and the family name itself is derived from 

 the word mactra, which signifies a kneading trough. In this group 

 the shells are all more or less triangular in form, with the valves 

 equal, and are either closed or very slightly gaping. The ligament, 

 perhaps more correctly designated the cartilage, is generally internal, 

 and contained in a deep triangular hollow ; and the shell is covered 

 with epidermis. The mantle of the animal is open in front, and 

 the siphonal tubes are united and fringed. The foot is usually large 

 and flattened. 



The typical genus, Mactra, contains some common molluscs that 

 bury themselves just beneath the surface of sandy beaches; and 

 these are so abundant in some parts of Great Britain that they are 

 used largely for feeding pigs. Some of the mactras are remarkable 

 for the great power and extensibility of the foot, which, in some cases, 

 is used so vigorously that the animal turns itself quickly over, or 

 even leaps on the ground. 



Our example of this genus is M. stultorum, which is a very 

 common object of the shore. Its colour is very variable, usually 

 some shade of grey or brown, and marked by radiating white lines. 



The Otter shells (Lutraria), of which we figure one species, are 

 much like the Mactrce in structure, and are usually included in the 



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