MODIOLA. 189 



iiKKlcratoly prominent, not curving outwards, and nearly in con- 

 tact very near the anterior extremity, which is small and rounded, 

 and the shell is nnich compressed at this part ; the lower margin 

 is generally curved or arched upwards, and gaping before the 

 mitldle for the passage of the byssus ; hinge-margin straight, and 

 ascending for about two thirds the length of the shell, so as to give 

 it additional height, then, by a regular downward curve, it produces 

 an obliquely rounded termination to the shell ; a broad, elevated 

 ridge crosses obliquely from the beaks to this termination, above 



Fig. 486. 



M. plicatida. 



■which the shell is compressed ; surface ornamented with numerous 

 I'adiating, somewhat undulating, occasionally branching ribs, most 

 conspicuous al^ive and behind, very fine on the anterior third. 

 Shell silvery-white, rather Ijrittle, covered with a thin, varnished 

 epidermis, variegated with yellow, green, and scorched colors, 

 usually arranged in zones ; stages of growth conspicuous ; within 

 silvery-white, the muscular impressions and margins of a livid 

 color ; margin of the posterior half and anterior side crenulated by 

 the ribs. Length, three inches; height, one and three twentieths 

 inches ; breadth, nine tenths of an inch. 



Inhal)its the tide waters of small streams where there is some 

 admixture of fresh water ; and also the drains in salt marshes. 

 In these localities they arc found crowded in among the stones of 

 the bed of the stream, or iml)edded in the peat-like soil of the 

 banks, near high-water mark. In this position, with the upper pos- 

 terior portion slightly exposed, they crowd in such numbers as to 

 form a complete stratum from six to twelve inches in thickness. A 

 great portion of the time they are, of course, out of water ; but 

 they retain enough to serve the demands of their economy during 

 the recess of the tide, and eject it when any disturbance prompts 

 them to close their shell. 



