336 TRIMERELLID&. 
MonoMERELLA, Billings, 1871. 
Distr.—Upper Silurian; Canada, Europe. J. prisca, 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. 
Dinopouus, Hall, 1871. 
Syn.—Rhynobolus, Hall (part), 1871.  Obolellina, Billings 
(part), 1871. Conradia, Hall, 1874. 
Distr.—Upper Silurian; N. Am., Europe. D. Davidsont, 
Salter. D. Conradi, Hall (exl, 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 
2 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.  Qbolellina, Billings 
(part), 1871. ? Gotlandia, Dall, 1871. 
Disir.—i sp. Silurian; Canada, U.S. TJ. Lindstromi, Dalm. 
(exdeso=s8i(*: 
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 elev: ated, 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 slightly raised; median plate cecurring generally in 
both valves, largest in the brachial one. 
