Bkkde.] Carboniferous Invertebrates. 141 



of the shell when these are not worn away, and also by the 

 convexity of the left valve and the small angle formed by the 

 hinge line and anterior margin (exclusive of the beaks). 



Myalina? exasperata. Plate XIX, fig. 4. 



Myalinaf exasperata Beede, Kans. Univ. Quart., vm, p. 128, pi. xxxn, 

 f. 1, (1899). 



Shell cuneate-ovate in outline ; beaks pointed, terminal ; shell 

 very thin, apparently composed of a single layer, compressed; 

 valves nearly or quite equal. The anterior? margin nearly 

 straight above, and merging into the narrowly rounded ventral 

 region ; the posterior? region similar to the anterior, but more 

 oblique. The surface is granular and marked by indistinct, 

 rather broad, concentric striae. Height, 43 mm. ; length, 28 

 mm. ; convexity, 3 mm. ; length of hinge, about 29 mm. 



Uange and distribution : Upper Coal Measures ; Topeka. 



The hinge of this shell is not sufficiently well shown to per- 

 mit of its proper classification. It is left in Myalina for the 

 present, for want of better information concerning its beak and 

 muscular impressions. The extreme thinness of the shell makes 

 it very probable that it does not belong to that genus. 



Myalina perattenuata. Plate XVI, fig. 8. 



Myalina y>< rattenuata Meek and Havden, Trans. Albanv Inst., iv, p. 77, 

 '(1858); Pal. Upp. Mo., p. 32, pi. i, ft. 12a, b, (1861). 



Meek and Hayden's description: "Shell very thin and frag- 

 ile, obliquely elongate, narrow and slightly arcuate ; valves 

 convex anteriorly, and compressed behind. Beaks pointed, 

 terminal, and attenuate ; hinge line equaling rather more than 

 half the entire length of the shell, and ranging at an angle of 

 about fifty degrees above the oblique anterior margin. Posterior 

 border sloping from the end of the hinge, nearly parallel to the 

 anterior side above, and rounding to the narrow an tero- nasal 

 extremity below ; anterior margin of the valves a little arcuate, 

 and rather abruptly deflected inward from the umbonal ridge 

 above the middle, and in outline nearly straight below. Um- 

 bonal slopes prominent from the beaks down the anterior side. 

 Surface with obscure subimbricating marks of growth. Length, 



