UNIONID^ AND MYTILID^. 93 



comforts of the voyage to the country of his adoption^ 

 the Lreissena is now contented and happy, and feels 

 himself quite at home among us. 



III. MoDioLA. — In Modiola the front of the sheU protrudes 

 a little beyond the apex, and forms a rounded termi- 

 nation, in this respect differing from Mytilus. The 

 shell of 



M. modiolus is large, full, covered with a brown epidermis, 

 not rayed ; of 



M. tulipa is smaller, narrower with rays of colour ; of 



M. phaseoUnaj small, oval, with a fringed epidermis ; 



M. barbata, larger, with a fringed epidermis. 



lY. Crenella differs from Modiola in the shell, which is 

 more oval, has a crenulated hinge-margin, and is ra- 

 diately grooved before and behind. The shell of 



C. nigra is rather flat, an inch long, and black ; of 



C. costulata, large at the back, with strong radiating 

 grooves at each end, and marbled with zigzag lines ; of 



C. rliomba, small, almost three-cornered ; of 



C. decussata, small, nearly square, crossed with strise ; 



C. 7narmorata, oblong, nearly straight, marbled, smooth in 

 the middle; of 



C. discors, like marmorata, but oblique; not marbled, 



