126 INVERTEBRATA OF MASSACHUSETTS. 



State Coll., No. 161. Soc. Cab., No. 1977. 



My'tilus demissus, Dillwyn ; Catal., i. 314. Wood; Index, pi. 12, f. 30. 



Modiola plicatula, Lam. ; Jin. sans Vert., vii. 22. 



My'tilus plicatulus, Deshayes ; Encyc. Mtth., Vers, ii. 368, pi. 220, f. 5. Sowee- 



Bv ; Genera, f. 7. 

 Modiola semi-costata, Conrad ; Journ. Acad. JVat. Sc, vii. 244, pi. 20, f. 7. 



Lister; Conch., pi. 353, f. 196. 



Shell transversely oblong-ovate, much elongated, narrow before 

 and widening backwards, somewhat falciform or arched ; beaks 

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

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

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

 is generally curved or arched upwards, and gaping before the 

 middle 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 

 which the shell is compressed ; surface ornamented with numer- 

 ous radiating, somewhat undulating, occasionally branching ribs, 

 most conspicuous above and behind, very fine on the anterior 

 third. Shell silvery-white, rather brittle, 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 cren- 

 ulated by the ribs. Length 3 inches, height l/g- inch, breadth 



j\ inch. 



Inhabits 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 are found crowded in among the stones of 

 the bed of the stream, or imbedded in the peat-like soil of the 

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

 posterior portion slightly exposed, they crowd in such numbers 

 as to form a complete stratum of six to twelve inches in thickness. 

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

 but ihey 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. 



