CONSTRUCTION OF SHELLS. 9 
shapes, and surrounded with narrow belts or margins, 
covered with scales. The shields of the Pholades 
bristle with points resembling a file, by means of 
which they are defended from external injury, when 
occupied in slowly excavating the hardest substances ; 
and the hospitable mansions of the peaceful Pinne, 
are sufficiently large for the admission of their un- 
assuming guests. The conical shaped shells of the 
Patellz (limpets) remind the traveller, of Anchorites 
dwelling by the sea-side in solitary tents: the 
Helices, or Snails, slowly perambulate the garden 
walks, with coverings that resemble the awnings of 
broad-wheeled waggons ; the Cardia, or Cockles, are 
provided with thick coatings, which enable them to 
endure the rough beating of a boisterous sea; and the 
shells of such species as are fragile, and transparent, 
and scarcely able to resist the slightest pressure, are 
found in still ponds and muddy ditches. A consi- 
derable number of Bivalves resemble little boats, and 
float upon the billows with their shells expanded to 
catch the breeze; others may be compared to vessels 
of a larger size, and are provided with sails, pumps, 
cordages, and every requisite for navigation. Such 
are those of the Pectines, Nautili, and Argonautz of 
the English, Mediterranean, and Indian seas. Others 
again, as the Mytilus, or Pearl Muscle, are inclosed 
in diving bells of an oblong or compressed form, with 
