STRUCTURE OF THE MOLLUSCA. 203 



The Myina have the mantle closed beneath, open in front 

 for the foot, and forming behind two long, united or sepa- 

 rated tubes; the foot small, compressed, tapering; the shell 

 elliptical, oval, or oblong, generally inequivalve, gaping at 

 both ends; the hinge with a concave prominent tooth in 

 one valve, and a corresponding depression in the other, for 

 the ligament. Mya, Lyonsia, Montacuta, Corhula. 



The Pholadina, remarkable for boring into stone, clay, or 

 wood, have the mantle closed and more or less tubular, with 

 an anterior aperture for the foot, which is small, and two 

 elongated coherent tubes behind; the branchise elongated, 

 so as to extend into the siphon ; the shell generally bare, 

 white, brittle, open at both ends, with the hinge toothless, the 

 ligament external, often with a calcareous plate. Pholas. 



The Teredines, which are remarkable for boring into 

 wood, have their mantle prolonged into a tube much longer 

 than their small rhomboidal valves, and terminated by two 

 very short tubes, of which the base is furnished on each 

 side with a hard and mobile plate. They are very destruc- 

 tive to the timber of harbours, and the planks of ships. 



The Fistulance, Gastrochcence, Clavagellce, and Aspergilla^ 

 belong to the same series. 



CLASS Y.-BEACHIOPODA. 



The Brachiopoda, fig. 6, are in some respects very similar 

 to the Lamellibranchiata, they being equally furnished with 

 a two-lobed mantle, and a bivalve shell ; but they are des- 

 titute of a foot, and present, in place of that organ, two 

 fleshy arms, fig. 8, furnished with filaments, and capable of 

 being extended and drawn within the shell, in a spiral form. 

 The branchiae are not distinct from the mantle, and the 

 mass formed by their viscera is very small. They have no 

 organs of locomotion, but live fixed to submarine bodies. 

 The principal genera are Lingula, Terebratula, and Orbicula, 



