14 ANADONTA. 



shell. Their composition is much the same as that of 

 the inner portion of the shell, being nacreous, and pos- 

 sessing a dull lustre. Some naturalists suppose they 

 are caused by particles of sand, or other irritating sub- 

 stances, getting into the stomach, or other organs; and, 

 the animal to remedy the annoyance, surrounds them 

 with a coating of pearl. Others aiErm that they are 

 the products of disease, and correspond to the calculi 

 in some animals. 



The average length is from four to five inches, and 

 about two in breadth. 



Hah. Most frequently found in mountain streams, 

 hence they are abundant in Wales, Cumberland, Lan- 

 cashire, the north of Yorkshire, and the hilly parts of 

 Cornwall and Devon. The Linn, Eden, and Yorkshire 

 Esk abound with them. 



ANADONTA. Brugiere. 

 Shell — Ohlong, equivalve, generally thin in 'proportion to the 

 size ; inequilateral ; hinge without teeth, hut furnished 

 with a lamina. 



Animal — Much the same as Unio. 



A. CYGNEA. Linnceuz. PI. II, ^^. 1. 

 Perhaps no shell illustrates the change of form and 

 appearance, which the same species exhibits, from being 

 matured under different circumstances as regards climate, 

 geological formation, and the nature of its food, more evi- 

 dently than the present. By some writers it has been 

 divided into a number of species, whilst others have en- 

 deavoured to classify the different forms under several 

 varieties. The former arrangement is, however, quite 

 exploded, while in adopting the latter, we own it offers 

 many difficulties. 



The normal form is elongated oval, moderately ven- 

 tricose, not very solid ; the lines of formation at pretty 

 regular distances; posterior end, not beaked, but round- 

 ly attenuated. 



The colour of the epidermis is olivaceous yellow 

 green. 



