MAE1NE MOLLUSCS 219 



such a number of stones &c. that the shell is completely hidden in 

 the entangled mass. Other British species include M, barbata, so 

 called on account of the peculiar fringed threads of the epidermis ; 

 M. phaseolina, in which the epidermis threads are not fringed ; 

 and M. tulipa, named from the streaks of crimson or purple that 

 radiate from the urnbones of the shell and remind us of the colour- 

 ing of the tulip flower. 



An allied sub-genus (Crenella) includes a few small British 

 molluscs the shells of which are crenulated on the dorsal margin 

 behind the ligament. The shells are short and swollen, and lined 

 by a brilliant pearly layer. One species (C. discors) is pale green, 

 with radiating lines from umbo to margin. It is common on 

 many of our shores, but is not easily found, as it hides at or below 

 low water mark, in a nest formed by binding together small stones. 

 Other species, one of which is black, are less abundant, and are 

 not readily obtained except by the use of the dredge. 



Before leaving this family we must refer to the remarkable 

 Dreissena polymorpha, sometimes called the Chambered Mussel, 

 on account of the chamber which 

 is formed in the beak of the 

 shell by means of a pearly plate 

 that stretches across it. This 

 animal is not indigenous to 

 Britain, but was introduced 

 from the East by trading vessels, 

 either attached by its silken Fio. 151. Dreissena polymorpha 

 byssus to timber that had been 



left floating in water previous to being shipped, or to the bottoms 

 of the ships. It seems to thrive almost equally well in salt, 

 brackish, and fresh waters, and has spread very rapidly since 

 its introduction. It is more commonly found, however, in docks, 

 canals, and rivers, and is on that account usually described with 

 the fresh-water species. 



The form of the shell is very similar to that of Mytilus, but has 

 no internal pearly layer, and the valves are bluntly keeled. The 

 mantle is closed, the siphons short, and the foot small. 



Our next family the Aviculidce contains those shells that are 

 distinguished by peculiar flat processes on each side of the um- 

 bones, one of which, the posterior, is generally wing-like in form. 

 They are popularly known as "Wing Shells, and the family 

 includes the so-called Pearl Oysters. Most of the species are 



