14 GENERA OF SHELLS. 



SECT. II. CONCH1FERA TENUIPEDA. 



Lateral gaping inconsiderable. 



Div. I. LIGAMENT INTERNAL, WITH OR WITHOUT 



AN EXTERNAL LIGAMENT. 



MACTRACEA. 



Shell equivalve, mostly gaping at the lateral extremities ; lig- 

 ament internal, with or without an external one. 



I. LIGAMENT ALWAYS INTERNAL. 

 (1.) Shells gaping at the sides. 



LUTRARIA. Shell inequilateral, transversely 

 oblong or rounded, gaping at the lateral ex- 

 tremities ; hinge with one tooth plaited to 

 resemble two, or two teeth, one simple, with 

 an adjacent deltoid-oblique pit projecting in- 

 wards ; lateral teeth none ; ligament interior, 

 fixed in the cardinal pits. 



Lutraria is distinguished from mactra, by having 

 no lateral teeth. 



(a.) Shell transversely oblong. 

 L. solenoides,. eUiptica, rugosa, lineata. 



(b.) Shell orbicular and subtriangular. 

 L. compressa, piperita, tellinoides, Candida, papyracea,, 

 plicatella, crassiplica, eanaliculata. 



MACTRA. Shell transverse, inequilateral, 

 subtriangular, gaping a little at the sides, beaks 

 prominent ; cardinal tooth on each valve com- 

 pressed, grooved, with a projecting fossa adja- 

 cent ; two lateral teeth near the hinge, com- 

 pressed, entering ; ligament interior, inserted 

 into the cardinal fossa. 



