MANUAL OF THE MOLLUSCA. 



Family IY. — Arcade.' 



Limopsis. Sub-genus, Trigonoccelia, Nyst. 



Shell approaches Leda in form, and differs from Limopsis in 

 the absence of the expanded ligamental area. 



Fossil, 7 species. Eocene. Paris basin, Belgium, England, 

 United States. 



Ctenodonta, Salter, 1851 (p. 427). 



Type, Tellinomya nasuta, Hall. 



Synonym, Tellinomya, Hall. 



Shell closed, differs from Isourca in not having the ligamental 

 area, the ventricose character, large and often subspiral beaks ; 

 the surface of the shell is smooth or marked by lines of growth, 

 but never cancellated ; hinge teeth small and numerous. 



Fossil, 40 species. Silurian — Carboniferous. Europe, N. 

 America, Bolivia. 



It is probable that most of the Palaeozoic species referred to 

 Nucula belong to Ctenodonta. 



Paejearca, Hall, 1858. 



Synonyms, Megalomus, Hall, 1852 ; Cyrtodonta, Billings, 

 1858; Cypricardites, Conrad, 1841. 



Example, C. Canadensis, Billings 



Shell equivalve, inequilateral; umbones near the anterior end 

 or terminal ; general form obliquely tumid, transversely sub- 

 rhomboidal ovate ; posterior extremity larger than the anterior, 

 and usually broadly rounded; two to eight oblique anterior 

 teeth beneath, or a little in front of the umbones ; two to four 

 remote lateral teeth parallel. with, the hinge line ; pallial line 

 simple ; muscular scars two, anterior sometimes deeply exca- 

 vated ; posterior superficial; ligament external. 



Some species have a narrow area between or behind the 

 beaks. 



Distribution, 42 species. Silurian — Devonian. N. America 

 and N. Wales. 



Sub-genus, Megambonia, Billings, 1858. 



Synonym, Vanuxemia, Hall, 1858. 



Shell ovate, beaks terminal, or nearly so ; anterior extremity 

 reduced to a small auriculate expansion, or obsolete. 



Distribution, 11 species. Silurian. N. America. 

 * See p. 424. 

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