Matheria] LAMELLIBEANCHIATA. 563 



by the muscular scar. Beaks of moderate, size and fullness, incurved, the anterior 

 half slightly flattened in the- casts. This flattening, which is j)roduced 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 

 muscQlar scar somewha-t uniform, not as sharply defined below as is usual for the 

 genus, very distinct, however, and partly overlapped above by the filling 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 (*. e., 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 diflficulties will have been overcome, since the Vanuxemia may 

 then be distinguished at once from the Cyrtodonta by the much greater distinctness 

 and character of the anterior muscular scar. (Comp. figs. 3 and 42 on plate xxxix.) 



Formation and localityT—Tv&nlOQ limestone, Minneapolis, Minnesota. 

 Mus. Reg. No. 5100. 



Genus MATHERIA, Billings. 



Matheria, Billikgs, 1858. C^. Nat. and Geol., vol. ill, p. 4i0. 



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. M. tenera is from 

 that horizon in Canada, one or two undescribed species occur in Kentucky, and M. 

 rugosa in Minnesota, while the Modiolopsis recta Hall, which is a true Matheria, 

 belongs to the Niagara of Wisconsin and Illinois. 



Matheria rugosa Ulrich. 



PLATE XXXVl, FIGS. 29 and 30. 



Matheria rugosa Uleich, 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 



