BIVALVE SHELLS. 39 



a cream yellow 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 few exceptions, inhabit the ocean only ; they, in 

 general, burrow just under the surface of the sand, being but barely 

 covered, owing to the shortness of their tube, by which they draw in 

 and throw out the water. They are used as a wholesome and nourish- 

 ing food ; and abound on most sandy shores. 



Genus 8 — MACTRA. 



Animal a Tethys; shell bivalve, with unequal sides, equi- 

 valve ; 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 ellip- 

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

 smooth, with irregular concentric striae, 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 are never found in fi-esh water, but lurk in the 

 sand or sludge, most commonly in the estuaries of rivers, or near 

 them. 



Genus 9.— DONAX. 



Animal a Tethys ; shell bivalve, with generally a creiiu- 

 late margin, the frontal margin very obtuse; hinge with two 

 teeth, and a single marginal one placed a little behind, rarely 

 double, triple, or none. 



Donax Trunculus — The Truncated Donax. Plate 

 IV. fig. 6, and Plate VI. fig. 9. Transverse, oblong, glossy, 

 finely striated longitudinally, and transversely banded and 

 radiated with purple ; white, clouded with purple within ; 

 internal margin crenulated. Inhabits the British coasts. 



The shells of this genus are ti*ansverse, triangular, inequilateral, flat- 

 tened, truncated before, and wedge-shaped. They inhabit the ocean ; 



d2 



