336 TRIMERELLID^. 



MONOMERELLA, Billiiigs, 18T1. 



Distr. — Upper Silui'ian ; Canada, Europe. M. pHsca, Billings 

 (cxxxix, 81, 82). 



Shell thick, circular or transversely oval ; large valve with pro- 

 jecting umbo, double-chambered ; area and deltidium large ; 

 hinge generally thick and elevated, ledge-shaped, depressed in 

 the middle ; cardinal facet a wall-like space rising out of or 

 behind the ledge of the hinge ; cardinal buttress strong, lamelli- 

 form ; platform flat ; slightly elevated ; widest, highest and very 

 obtusely angulated in front ; in the brachial valve the platform 

 is trilobed, usually with a thin margin. 



DiNOBOLUs, Hall, 1871. 



Sijn. — Rhynobolus, Hall (part), 1871. Obolellina, Billings 

 (part), 1871. Conradia, Hall, 1874. 



Distr. — Upper Silurian; N. Am., Europe. D. Davidsoni, 

 Salter. D. Conradi, Hall (cxl, 83, 84). 



Shell circular, or transversely oval, moderately thick ; pedicel- 

 valve with slightly prominent umbo ; area wider than long ; plat- 

 form more or less sinuated ; widely V-shaped and slightly raised 

 in front ; crescent prominently marked in its crown and sides ; 

 hinge moderately thick, with a rounded edge on which, and in 

 front of the cardinal facet, is a pair of scars ; brachial valve 

 rather tumid at the umbo, somewhat strongly trilobed, outer 

 margins a little raised, antemedian portion rounded, projecting, 

 and terminating in a slightly developed median plate ; crescent 

 a strongly marked linear scar on the hinge, arching forward in 

 front of the cardinal facet , inner border of its sides with strongly 

 marked indentations, outer border a fine line ; a rather strongly 

 marked subcardinal scar in the umbonal cavity ; a large rhom- 

 boidal postmedian scar in front of the latter. 



Trimerella, Billings, 1862. 



Syn. — Rhynobolus, Hall (part), 1871. Obolellina, Billings 

 (part), 1871". ? Gotlandia, Ball, 1871. 



Distr. — 7 sp. Silurian; Canada, U. S. T. Lindstromi^Dalm. 

 (cxl, 85-87 \ 



Valves thick, longitudinally oval ; umbo of pedicel-valve 

 usually massive, solid, occasionally double-chambered, irregu- 

 larly projecting; area of considerable size, longer than wide; 

 deltidium large; hinge generally thick and elevated, rudely or 

 slightly dentary, and variously modified in different species ; 

 cardinal facet large ; crescent rather well-defined in typical 

 species; platforms elevated, and doubly vaulted, occasionally 

 solid and slightl}^ raised ; median plate occurring generally in 

 both valves, largest in the brachial one. 



