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under the beaks. Facet of the ligament marginal, narrow, channelled, not 

 traversed by the byssus. 



A. macroptera, A. falcata, A. squamulosa, A. physoides, 



' rotorium, ' crocea, ' papilionacea, ' virens, 



1 semisagitta, ' Tarentina, ' costellata, 2 fossil species. 



' heteroptera, ' Atlantica, 



Meleagrina. Shell subequivalve, rotundate, nearly square, externally 

 squamose; the inferior cardinal margin straight, not caudate anteriorly; a sinus 

 at the posterior base of the valves, for the passage of the byssus; the left 

 valve being at this place narrow and channelled; hinge linear, without teeth. 

 Facet of the ligament marginal, elongated, nearly external, dilated in the 

 middle. 



M. margaritifera, M. albina. 



SECTION II. Ligament not marginal, contracted into a short space under the 

 beaks; always visible, and not forming a tendinous cord under the shell. 



This section is divided into two families, viz. Pectinida and Ostracea. 



PECTINIDA.— Seven Genera. 



Ligament internal, or demi-intemal. Shell in general regular, compact, 

 not foliated. 



Pedum. Shell inequivalve, subauriculated, lower valve gaping; beaks un- 

 equal, distant; hinge without teeth. Ligament partly external, inserted in an 

 elongated and channelled pit, formed in the lower side of the beaks. Lower 

 valve notched near the posterior base. 

 P. spondyloideum. 



Lima. Shell longitudinal, subequivalve, auriculated, gaping slightly on 

 one side between the valves; beaks distant; their internal facet inclined out- 

 wards; hinge without teeth. The cardinal pit partly external, receiving the 

 ligament. 



L. inflata, L. glacialis, L. fragilis, 5 fossil species. 



' squamosa, ' annulata, ' linguatula, 



Plagio stoma. Shell subequivalve, free, subauriculated, the cardinal base 

 transverse, straight; beaks rather distant, their inner sides expanding into 



