MOLLUSCA. 



Unio Pictorum. a Head of the shell, p TaU, with tw« 

 tubes. B Hinge, r Foot. 



Noah's Ark. Fig. 1 Side view. Fig. 2 The shell, with 

 the hinge and umbones presented. Fig. 3 A single 

 yalve, showing the hinge, a a The umbones. b The 

 margin. 



Cyclas Cornea. 



The class entitled Conchifera, are mostly bivalves, though a few of its species are multivalves, and a 

 few others of completely anomalous configuration. They are acephala, or headless, the entrance to the 

 stomach being buried between the folds of the mantle. This class is best distinguished by the shell, 

 which is composed of particles of lime exuded from the surface of the mantle, in combination with a 

 gluey secretion which holds them together. Each valve consists of a number of layers, of which the 

 outermost is the smallest, each inner one projecting beyond the one covering it. 



The valves are connected by hinges of various kinds, together with an elastic ligament, which serves at 

 once to bind together and to keep them a little apart, which is their natural position. If the animal 

 wishes to draw the valves tightly together, it does so by means of the adductor muscle, which is attached 

 to the interior of both valves. 



Many of this class have a sort of foot, a fleshy, muscular organ, used for locomotion, and for sundry 

 purposes besides, though in others it is wanting. 



The Cyclas Cornea is a bivalve. The mantle, which belongs to all the various species of the Conchifera, 

 is, in this animal, prolonged posterioi'ly into a twofold tube, or syphon, one branch of which is designed to 

 admit the water, which contains the creature's food, as also the oxygen, by which its blood is renovated, 

 and the other serving to carry off the execrementitious matters. Through another aperture in the mantle, 

 it protrudes at will, its large, fleshy foot, which serves not only for locomotion, but for scooping out a 

 /etreat in the sand or mud, where it sometimes burrows, projecting its double syphon from the mouth of its 

 hole, for the twofold purpose of breathing, and of discharging waste matter. 



The Unio Pictorum is a fresh-water bivalve, and though larger than the Cyclas Cornea, and differently 



shaped, the general particulars of its structure are so closely analogous, that we need not here repeat them. 



The observer will note, in the plate, that a is the forward extremity of the shell ; p the hinder extremity, 



with the two syphons there visible ; h the hinge uniting the valves ; and f the foot, extended nearly to 



he full. 



The Noah's Ark, a bivalve, is a native of the Atlantic Ocean, and the European seas. It differs not, in 

 its essential characteristics from the two already described. The shell is boat-shaped, rather thick, equi- 

 valve. though not equi-lateral ; the form is elongated, and somewhat oblique; the umbones are distant, 

 frequent!)- a little curved forward; the hinge is straight-lined, with numerous small interlocking teeth, and 



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