LOWER SILURIAN LAMELLIBRANCHS. 675 



Ischyrodonta elongata, Ulrich. 



Plate 54. Figs. 20 and 21. 

 Ischyrodonta elongata, Ulrich, 1890, American Geologist, Vol. VI, p. 175. 



Shell large for the genus, trail sversely elongate-ovate, widest pos- 

 teriorly, strongly convex, with point of greatest convexity a little in 

 front of the center. Beaks rather large, compressed, almost terminal; 

 umbonal ridge rather strong, mesial sulcus broad. Cardinal margin 

 strongly arcuate; posterior margin nearly vertical in the middle and 

 lower part, uniformly rounded above, sharply curved at the base; ven- 

 tral margin faintly and broadly sinuate; anterior end very short and 

 abruptly rounded. Surface marked with strong and irregular lines of 

 growth. 



In casts of the interior the beaks are very prominent, incurved, and 

 greatly compressed hy the deep umbonal sulcus, behind which the some- 

 what curved and sharply elevated umbonal ridge is distinguishable al- 

 most to the basal line. Anterior muscular scar deep, subrhomboidal, 

 sharply defined on the upper side, radially marked and situated immedi- 

 ately beneath the beak. Just above it is the small pedal muscle scar. 

 Posterior scar faint, ovate; pallial line distinct in the basal part of the 

 valves. 



This fine species, though clo-ely related to I. truncata, is readily 

 distinguished by its much greater length. The next species is also re- 

 lated, but is smaller and widely different in the post-cardinal part of its 

 outline. 



Formation and Locality. — Near the top of the Cincinnati group, at 

 Oxford, Ohio, and Richmond, Indiana. 



Ischyrodonta miseneri, li. sp. 



Plate 54. Figs. 10 and 11. 



This species, as far as known, is very similiar to /". elongata, and a 

 detailed description is scarely necessary. Though agreeing in most re- 

 spects very closely with that species, a comparison still brings out differ- 

 ences that doubtless will suffice in discriminating between the two 

 species. The shell of /. miseneri is considerably smaller (the largest 

 seen is but 38 mm. in length), comparatively a little shorter, and sub- 

 triaugular in outline. The posterior margin is more oblique and con- 

 siderably higher, and its junction with the straight cardinal margin 

 angular, while the post-cardinal region is distinctly alate and thus quite 

 different from the rounded and sloping character of this part of the out- 

 line in /. elongata. The umbonal ridge furthermore is a more decided 

 feature. Of the other species I. decipiens is much shorter, and I. 

 modiolifo) mis longer, and more produced and obliquely rounded posteriori}^. 



