52 THE ELEMENTS 



or sludge, most commonly at the 

 mouths of rivers, or near them. 



The French naturalists divide the 

 Mactra into two genera ; Mactra and 

 Lutraria. 



In Turton*s Linne twenty-seven 

 species are described ; twelve have 

 been discovered in the British seas. 



Genus 9th. — DONAX. Animal a 

 Tethys ; shell bivalve, with generally 

 a crenulate margin, the frontal margin 

 very obtuse ; hinge with two teeth, 

 and a single marginal one placed a 

 little behind, rarely double, triple or 

 none. Linn. Syst. 308. Plate 6th, 

 fig. 9th. 



The shells of this genus are in ge- 

 neral triangular, inequilateral, flatten- 

 ed, treuncated before, and wedge- 

 shaped. 



Habitation. The Donax has only 

 been found to inhabit the ocean ; 

 lurking in the sand of the shore, and 



