BIVALVE SHELLS. 33 



pink ; half an inch long and three fourths of an inch broad. In- 

 habits the coasts of Britain. 



The Tellinse inhabit the ocean, rivers and lakes. Those of the ocean lie 

 buried in the sludge, near the shore, and make two apertures in the sand ; 

 when the tide lias receded, they may be easily found by looking for these 

 marks. 



Genus 7 — CARDIUM. 



Animal a Tethys; shell bivalve, nearly equilateral, equivalve, 

 generally convex, longitudinally ribbed, striate or grooved, the 

 margin dentated ; hinge with two alternate teeth in the middle, 

 near the beak ; one of tliem commonly incurved ; and larger 

 remote lateral teeth on one side, each locking into hollows in 

 the opposite valve 



Cardium edule. — The Edible Cardium. Piute V. fig. 7. 

 With about twenty-six, antiquated longitudinal ribs j of a cream 

 yellov/ colour ; the posterior end a little elongated. Usual 

 diameter one inch ; but they are found in Orkney two and a 

 half inches in breadth, and two and a-fourth in length ; in which 

 case the posterior extremity is considerably produced. When 

 young the cockle is nearly orbicular, and pure white. 



The cockles, with the exception of one species, inhabit the ocean only ; 

 they, in general, burrow just under the surface of the sand, barely covered, 

 to which. they are necessitated from the shortness of their tube, by which 

 they draw in and throw out the water. They are used as a wholesome and 

 nourishing food ; and abound in most sandy shores. 



Genus 8.— MACTRA. 



Animal a Tethys ; shell bivalve, with unequal sides, equivalve ; 

 middle tooth of the hinge complicated, with a small hollow on 

 each side, lateral teeth remote, and inserted into each other. 



Mactra Lutraria. — The mud Mactra. (Lutraria elliptioa, 

 Lamarck.) Plate VI. fig. 8. Transverse oblong-oval, smooth, 

 with irregular concentric strife, and an olive, or yellowish 

 epidermis, glossy white within ; gaping at both ends. Two and 

 a-half inches long, and from four to six broad. Inhabits the 

 British coasts. 



Shells of this genus inhabit the ocean only. These lurk in the sand or 

 sludge, most commonly at the mouths of rivers, or near them. 



