Account of some remarkable Shells. 271 
one of these shells has hitherto had any mes claim to be 
considered as a native of Great Britain. — sedi 
The species which principally tini the notice . of the 
members present, and which, from its singularity, was the cause 
of the fragment of stone being presented to the society: by Mr. 
Sowerby, we find, after due examination, to be no other than 
the Mytilus lithophagus, of Linnzus,—a well known shell, and 
described very fully by many different authors, in consequence 
of its remarkable property of perforating rocks and other stony 
bodies. The circumstances which at first seemed to authorise 
its being considered a distinct species are two beak-like pro- 
cesses, (one from the extremity of each valve), apparently form- 
ing a part of the valves themselves, and crossing each other in 
some degree like the two mandibles of the bird called by Lins 
neus Lovia curvirostra, or Cross-bill. On separating the valves, 
however, these processes prove to be wholiy external and adven- 
titious, and the valves to be ** extremitatibus utrinque rotundatis," 
the words employed by Linnzus to characterize the true Mytilus 
lithophagus, which thus often acquires an inseparable calcareous 
coating, during the solvent process it employs for forming cavities 
in that kind of stone. The shell is very accurately figured with 
these artificial appendages in the 221st plate of the Tableau En- 
cyclopédique,. article Vers Testacées (fig. 8. a. 8. .) One or two 
of the specimens, however, differ from the ordinary Mytilus li- 
thophagus (the shape of which is pretty exactly cylindrical) in 
having a sort of angular protrusion on one side; this is a devia- 
tion from regularity of outline common among the Mytili, and 
does not appear to us to warrant the construction of a new spe- 
cies. It was noticed by Gualtieri, who, in figuring three speci-- 
mens of Mytilus lithophagus (tab. 90. PA D.) has repaid one 
el hefi form we allude to. Da 
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