LAMELLIBRANCHIATA. 563 
Matheria ] 
by the muscular scar. Beaks of moderate size and fullness, incurved, the anterior 
half slightly flattened in the casts. This flattening, which is produced by the usual 
internal thickening of the anterior part of the shell, extends obliquely backward 
and downward from the beaks toward the middle of the ventral edge. Anterior 
muscular scar somewhat uniform, not as sharply defined below as is usual for the 
genus, very distinct, however, and partly overlapped above by the fillimg of the 
beaks. Pallial line sharp in the anterior two-fifths, obscure behind. Posterior 
muscular impression too light to be determined with certainty. Hinge plate nar- 
row, with two slender horizontal posterior lateral teeth in the left valve and three 
in the right. Cardinal teeth unknown. 
This species is associated with several of Cyrtodonta that, under ordinary condi- 
tions, are not easily distinguished. The feature to be chiefly relied upon in separating 
them (i. ¢., the character of the anterior muscular scar) is usually obscured by crys- 
tallized remnants of the shell. When these are removed and a clean cast of the inteior 
has been produced the difficulties will have been overcome, since the l’anuxemia may 
then be distinguished at once from the Cyrtodonta by the much greater distinctness 
and character of the anterior muscular sca: ‘Comp. figs. 3 and 42 on plate xxx1x.) 
Formation and locality.—Trenton limestone, Minneapolis, Minnesota. 
Mus. Reg. No. 5100; 626, 
Genus MATHERIA, Billings. 
Matheria, BrttrNes, 1858. Can. Nat. and Geol., vol. iii, p. +40. 
Shell equivalve, very inequilateral, oblong quadrate or suboval; beaks small 
anterior. Surface marked with concentric growth lines only. Hinge of moderate, 
strength or rather weak, with external linear ligamental area posterior to the beaks, 
two small, divaricating cardinal teeth beneath the beak of the left valve and only 
one in the right; no lateral teeth. Adductor impressions two; the anterior one 
smaller and better defined than the posterior. Pallial line simple, obscurely defined. 
Type: Matheria tenera Billings. 
Only four or five species known to me have the characters of this genus. They 
are all small shells and with one exception belong to the Trenton. J. fenera is from 
that horizon in Canada, one or two undescribed species occur in Kentucky, and J, 
rugosa in Minnesota, while the Modiolopsis recta Hall, which is a true Matheria, 
belongs to the Niagara of Wisconsin and Illinois. 
Marueria ruGosa Ulrich. 
PLATE XXXVI, FIGS. 29 and 30. 
Matheria rugosa Utricu, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur., p. 241. 
Shell large for the genus, trapezoidal, widest posteriorly, with the beaks nearly 
terminal, small, incurved, projecting slightly above the hinge; a strongly convex 
