MODIOLARIA. 123 



is obliquely and finely crennlatecl : hinge reflected and deeply 

 indented : inside nacreous and iridescent, notched all round 

 the edge, except on the ventral side : muscular scars slight and 

 scarcely perceptible. L. 0-75. B. 0*45. 



Habitat : Imbedded in the skin or outer integu- 

 ment of Ascidia mentula and other simple Tunicata, or 

 attached by its byssus to old shells, in the Laminarian 

 and Coralline zones on all our coasts. It is by no 

 means rare ; and sometimes a score of specimens may 

 be extracted from a large Ascidia. In a fossil state it 

 occurs in the Red and Coralline Crag. The limits of its 

 foreign distribution comprise the sea-bed lying between 

 Finmark and the JEgean, and reach westward to the 

 Canaries. On the Norwegian coast Asbjornsen has 

 given 10, and Danielssen 150 fathoms as its bathy- 

 metrical range. It has not been quoted in any list of 

 Icelandic, or of North American shells. 



On being dislodged from its usual place of abode, 

 M. marmorata puts out its foot and feels its way in 

 search of another retreat ; and when it has found one to 

 its liking, it immediately spins a byssus and securely 

 fastens itself, sometimes on or within the fold of a 

 seaweed, or in the crevice of a stone. I have seen it (as 

 if acting on a sudden impulse, or disliking to be watched) 

 detach itself from its mooring and set out again on its 

 travels to select a more sheltered or suitable spot. The 

 genesis or development of the animal has been fully and 

 most ably elucidated by Loven. 



This species used to be called the Mytilus discors of 

 Linne; and it is not improbable that in his descrip- 

 tion he included it with the Mytilus discrepans of Mon- 

 tagu, which we now refer to Linnets species. But the 

 epithet " fusca " in the e Systema Natime ' is scarcely 

 applicable to the present species; and as one of the 



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