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jecting almost vertically; a cardinal pit in the other valve. Ligament in- 

 ternal, inserted in the prominent tooth and the corresponding pit. 



M. truncata, M. arenaria, M. erodona, M. solenimyalis. 



Anatina. Shell transverse, nearly equivalve, gaping at one or both sides; 

 one naked, broad, spoon-shaped, cardinal tooth, projecting internally in each 

 valve, receiving the ligament. In many species, a lamina falcated rib runs 

 obliquely below the cardinal tooth. 



A. latorna, A. longirostris, A. rugosa, A. myalis, 



' truncata, ' globulosa, ' imperfecta, ' rupicola. 



' subrostrata, ' trapezoides, 



SECTION II. CONCHIFERA TENUIPEDA. The lateral gaping, in- 

 considerable. 



The shells of this section compose four families, which are separated into 

 two divisions, viz. — I. Mactracea and Corbulea, having the ligament internal, 

 with or without any external ligament; and II. Lithophaga and Nymphacea, 

 having the ligament always external. 



DIVISION I. Ligament internal, with or without any external ligament. 



MACTRACEA.— Seven Genera. 



Shell equivalve, mostly gaping at the lateral extremities. Ligament inter- 

 nal, with or without any external ligament. 



1 . Shells gaping at the sides. 



Lutraria. Shell inequilateral, transversely oblong or rounded, lateral 

 extremities gaping; hinge with one tooth folded in two; or two teeth, one of 

 which is simple, with an adjoining, deltoid, oblique pit, projecting inwards; 

 no lateral teeth. Ligament internal, affixed in the pits. 



L. solenoides, L. compressa, L. Candida, L. crassiplica, 



' elliptica, ' piperata, ' papyracea, ' complanata. 



' rugosa, ' tellinoides, ' plicatclla, 



Mactra. Shell transverse, inequilateral, subtriangular, gaping very little 

 at the sides; beaks prominent. One compressed, folded, cardinal tooth in 

 each valve, with an adjoining pit, projecting inwards; two compressed, en- 



