Bkkde.J Carboniferous Invertebrates. 147 



three or four teeth on the anterior of the hinge which are but 

 slightly oblique, having a small angle with the long teeth of 

 the posterior part of the shell. The first three teeth are much 

 more oblique, and just back of them are six small, round teeth, 

 back of which are two or three teeth oblique in the opposite 

 direction to those on the front of the hinge. It differs from M. 

 carbonarius (Cox) in the area of obsolete striae, in having the 

 beak placed farther toward the rear, in having the coarse striae 

 in front, and the posterior margin curving farther forward to 

 meet the hinge. 



These specimens are from about the same locality as that 

 figured by Keyes as M. sangamonensis f Keyes's 11 figure agrees 

 exactly with neither M. sangamonensis as figured byWorthen, 

 nor entirely with our specimens, and, inasmuch as there is no 

 description given that will distinguish this from several other 

 species of the genus, I am in doubt as to whether his figure 

 represents a specimen more like M. sangamonensis or like ours ; 

 but he has the hinge line indicated as longer than the rest of 

 the shell and the umbonal ridge as nearly destitute of stria*, 

 while our specimens possess large striae at this place. 



It is with some difficulty that the young of these specimens 

 can be distinguished from M. tenuistriatus Meek, but the striae 

 are much heavier on the anterior than on the central portions 

 of the shell, even in the young specimens. The outline and 

 general appearance aside from this are very similar. 



Macrodon obsoletus. Plate XX, fig. 13. 



Macrodon obsoletus Meek, List Carb. Foss. W. Va., v, (Ex. Rep. Reg. 

 Univ. \V. Va.), (1871); Geol. Surv. Ohio, n, Pal., p. 334, pi. xix, f. 9. 



Shell oblong, elliptical, of medium size; hinge line about 

 equaling the length of the shell ; anterior margin making a 

 right angle, or sometimes less, with the hinge line, and round- 

 ing gradually into the ventral margin, which is a little sinuate 

 near the center and nearly parallel with the hinge line ; pos- 

 terior margin abruptly but regularly rounded, curving forward 

 to meet the hinge line. Sinus across the shell from the umbo to 



11. Sen. Surv. Mr.., V, p. 121. pi. xr.vr, f. 2. I 1- 



