BIANUAL OF THE MOLLUSCA. 
Faminy ITV.—Arcap2.* 
Lrvopsis. Sub-genus, Trigonocelia, Nyst. . 
Shell approaches Leda in form, and differs from Limopsis in 
the absence of the expanded hgamental area. 
Fossil, 7 species. Eocene. Paris basin, Belgium, England, 
United States. 
OTENODONTA, Salter, 1851 (p. 427). 
Type, Tellinomya nasuta, Hall. 
Synonym, Tellinomya, Hall. 
Shell closed, differs from Jsourca 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. 
PAL#ARCA, Hall, 1858. 
Synonyms, Megalomus, Hall, 1852; Cyrtodonta, Billings, 
1858; Cypricardites, Conrad, 1841. 
Exampie, 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 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; hgament external. 
Some species haye a narrow area between or behind the 
beaks. 
Distribution, 42 species. Silurian—Deyonian. 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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