266 TRANSACTIONS OF TUE [mAR. 15, 



about one-fourth, or less, the length of the shell from the an- 

 terior end. 



Hinge-line straight or slightl}- arcuate. Anterior margin of 

 the shell sub-semi-circular. Ventral margin nearly straight or 

 slightly arcuate, converging posteriorly towards the dorsal mar- 

 gin. Posterior margin rather sharply rounded at the most pos- 

 terior projection of the shell, which is about midway of the 

 height, arcuate above and below this point. 



Surface, anterior to the beak, ornamented with moderatel}' 

 fine, regular, concentric wrinkles, which become obsolete poste- 

 riorly. Shell smooth posterior to the beaks. 



This species differs from A. ivalkeri in its posteriorly converg- 

 ing dorsal and ventral margins, in the more sharply rounded pos- 

 terior extremity, in the sharper and more elevated beaks, and in 

 the finer and more regular concentric wrinkles which are con- 

 fined to the region anterior to the beaks. 



SCHIZODUS BATESVILLENSIS Sp. nOV. 



PL XX., figs. 8-9. 



Shell of medium size, longitudinally subovate in outline, 

 valves moderately convex. Beaks prominent and incurved, 

 situated at about one-third the length of the shell from the 

 anterior end. Greatest convexity of the shell immediateh' be- 

 low the beak, at about one-third the distance from the point 

 of the beak to the ventral margin. Umbonal ridge arcuate, 

 rounded and ill-defined, posterior umbonal slope concave. 



Hinge line straight, reaching about two-thirds the distance 

 from the beak to the posterior point of the shell. Anterior 

 margin regularly rounded. Ventral margin arcuate, curving ab- 

 ruptly upward posteriorly, to meet the posterior margin. Poste- 

 rior margin more or less arcuate, forming a rather abrupt angle 

 with the ventral margin at the most posterior point of the shell, 

 and meeting the hinge-line at a more or less distinct obtuse angle. 



Surface smooth, except on the anterior slope, which is orna- 

 mented with fine concentric lines, which can be seen only upon 

 the best preserved specimens and when held in a strong light. 



The shell is similar to and probably identical with the one de- 

 scribed from the Maxville limestone b}' Whitfield,* as S. chester- 

 ensis, M. & W. In his description he states, however, that it 

 may become necessary to separate the Maxville limestone speci- 

 mens from S. chesferensis as a distinct species. S. hatesvillensis 



*Am. N. Y. Acad. Sci., Vol. 5, p. 587, PL XIV., fig. 4. and Geol. Surv., Ohio, Vol. 7. 

 p. 475, PI. X., fig. 4. 



