LAMELLIBRANCHIATA. 351 



Three specimens measure respectively 31, 27 and 22 mm. in length, and 

 14, 15 and 10 mm. in height. 



This species is distinguished by its parallel basal and cardinal margins, the 

 truncated posterior end and its narrow form. It resembles M. parvus (White 

 and Whitfield), of the yellow sandstone of Burlington, Iowa, but it is a much 

 larger form and the beaks are more anterior. 



Formation and localities. In the Chemung group, at Buck's quarry, near 

 Elmira, at Philipsburgh, Rockville, Chemung creek and Leon, N. Y. ; also at 



Meadville, Pa. 



Macrodon ovatus. 



PLATE LI, FIG. 8 ; AND PLATE XCIII, FIG. 16. 

 Macrodon ovatus. Hall. Prelim. Notice Lamellibranchiata, 2, p. 15. 1870. 



Shell small, ovate, wider behind ; length once and a half the height ; basal 

 margin gently curving with a slight constriction anterior to the middle ; 

 posterior margin regularly rounded below and obliquely truncate above. 

 Cardinal line very gently arcuate. Anterior end short and narrowly rounded. 



Valves convex, very gibbous above the middle and in the umbonal region. 



Beaks sub-anterior, prominent, rising a little above the hinge-line. 

 Umbonal region gibbous with sometimes an undefined ridge extending 

 obliquely backward to the basal margin, anterior to which is a gentle con- 

 striction, (too strongly represented in the figure). 



Surface marked by concentric lines of growth, which appear to have 

 been lamellose; there is no evidence of radiating stride in the specimens 

 examined. 



Anterior hinge-teeth minute, oblique; posterior teeth elongate, very 

 slightly curved, one long and one short in each valve. 



Three specimens measure respectively 18.5, 15 and 15 mm. in length, and 

 12, 19 and 9 mm. in height. 



This species differs from M. Hamiltonia in its shorter form, gently curving 

 hinge, apparent absence of radiating striae, and in not having the regular con- 

 centric varices of that species. 



Formation and localities. In the Waverly sandstones, at Granville and New- 

 ark, O. 



