ANATINID^. 45 



T. convexa : large,, brownish, much swelled anteriorly, rather 

 angular, and square posteriorly ; very inequivalve, 



T. imhescens : much the same form, but whiter, smaller, less 

 inequivalve, and more square. 



T. johaseolina : thin, white, rather long, oval, oblique. 



T. villosiusGula : shorter than jpliaseolina, not oblique. 



T. distorta : short and squarish ; although not a borer, often 

 mistaken for one, through its habit of getting into 

 crevices when young, and shaping its shell according 

 to the irregularities of its position; hence it becomes 

 distorted : its beaks are sometimes tinged with purple. 



II. CocHLODESMA. — The only British species of this, C. 

 prcBtemds, is white, shaped like a Thracia, but is equi- 

 valve, and has in each valve a spoon-shaped process 

 jutting from the hinge. 



