34 CONCHOLOGY. 



versely oval, gaping at the extremities ; hinge with one car- 

 dinal tooth folded in two, or two teeth, one of which is plain, 

 with an opposite hollow to receive it ; no lateral teeth ; liga- 

 ment interior and fixed in the hollow cavity of the primary 

 tooth. 



Lutraria solenoides. Lutraria elliptica. 



L. rugosa. L. papyracea. 



L. compressa. L. plicatella. 



L. piperata. L. crassiplica. 



L. teUinoides. L. complanata. 



L. Candida. 



L. solenoides. The Solen-like Lutraria. 



Species oblong, sub-cylindrical, very gaping, two very 

 strong cardinal teeth ; the spoonlike cavity of the ligament 

 vertical. 



L. compressa. The compressed Lutraria. 



Species oval or orbicular, almost equilateral, very com- 

 pressed, little gaping ; hinge similar ; internal ligament in- 

 serted in the pit of a vertical spoonlike cavity ; two distinct 

 tubes, without longitudinal striag. 



L. rugosa. The rugged Lutraria. 

 Species ovate, closed at both ends ; striae from the summit 

 to the base. 



L. elliptica. The oval Lutraria. 



Oblong oval, nearly smooth, having a few concentric striae, 

 and some diagonal striae at the ends of the valves ; colour 

 yellow or greenish brown ; inside white. 



2. Mactra. The Kneading-trough. Thirty-three species. 



The name given to this genus was derived from the Greek 

 word fiaKTpa, from its resemblance to a trough used for 

 kneading bread. 



In all species of this genus a similarity of colouring and 

 form pervades the whole. In shape they are sub-triangular 

 or oblong, with a smooth, striated, or transversely-ribbed ex- 

 terior. In some species the valves gape at both ends, and in 



