MANUAIi OF THE MOLLUSCA. 



Family IY. — Abcadje.* 



LiMOPSis. Sub-genus, Trigonocoelia, Nyst. 



Shell approaclies 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. 



CTE]S;ODO]srTA, Salter, 1851 (p. 427). 



Type, Tellinomya nasuta, Hall. 



Synonym, Tellinomya, Hall. 



Shell closed, differs from Isourca in not haying 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. 



Pai^abca, Hall, 1858. 



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

 1858; Cypricardites, Conrad, 1841. 



Fxample, C. Canadensis, Billings. 



Shell equivaive, 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 Httle 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. 



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

 and N. Wales. 



Sub-genns, Megamlonia, Billings, 1858. 



Synonym, Yanuxemia, Hall, 1858. 



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

 reduced to a small auriculate expansion, or obsolete. 



Distrilution, 11 species. Silurian. N. America. 



* See p. 424. 



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