120 MYTILID^E. 



invests itself in a case of gravelly and shelly fragments. 

 The fry are of a rhomboidal shape. The size of adult 

 specimens does not appear to exceed three-quarters of 

 an inch, those from the south being much smaller than 

 northern ones. The shell resembles that of Modiolaria 

 in the denticulation of the dorsal margin. It differs 

 from the young of M. modiolus in having this peculiar 

 character, as well as in being much thicker, more con- 

 vex, of a darker colour, and angulated behind. 



Allied to M.phaseolinus in respect of the denticulated 

 hinge and dorsal margin is the M. crenatus of Lamarck, 

 which was imported in 1816 into Portsmouth harbour 

 on the bottom of H. M. S. Wellesley from Bombay; 

 but it has not become naturalized. M . bidens of Linne 

 and M. Africanus of Chemnitz were subsequently intro- 

 duced in the same manner, and with a like result, into 

 Scarborough and Plymouth. The former is a West- 

 Indian, and the latter a West-African species. Modiola 

 agglutinans of Cantraine (M. vestita, Philippi) is said to 

 have been once found in Ardmore Bay, near Youghal, 

 and was described by Captain Brown under the name 

 of Modiola Ballii. It is a Mediterranean species. The 

 Mytilus (Lithodomus) aristatus of Dillwyn was found 

 many years ago in a piece of ballast limestone imported 

 into this country from the West Indies, and was for- 

 merly included among British shells. 



Genus II. MODIOLA'RIA *, Beck. PL III. f. 3. 



BODY suboval : mantle folded in front into a wide incurrent 

 tube, and behind into a conical excurrent tube : foot strap- 

 shaped. 



SHELL rhomboidal, sculptured by two rows (one on each 

 side) of striae which radiate from the beaks, leaving the middle 



* Allied to the genus Modiola of Lamarck. 



