272 PALEONTOLOGY OF NEW YORK. 



the anterior. Anterior end comparatively narrow and abruptly rounded, 

 limited by a broad depression extending from the beak to the basal margin 

 about one-third the length of the shell from the anterior extremity. 



Valves very convex, gibbous in the middle and in the unibonal region ; 

 the greatest convexity is about the middle of the length, sloping toward 

 the posterior end more abruptly than toward the umbo. The depth of 

 l)oth valves is nearly equal to the height of the shell. 



Hinge-line oblique, equal to two-thirds the length of the shell. 



Beaks prominent, incurved, directed forward. Umbonal region gibbous, 

 rising considerably above the cardinal-line ; posterior slope sub-angular. 



Test thick, concentrically striated ; its special characters unknown. 



Muscular impressions large ; posterior impression situated on the cardinal 

 slope. 



The specimen described has a length of 105 mm., height 70 mm., and the 

 depth of both valves is about 60 mm. 



This species is distinguished by its rhomboid-ovate form, its great gibbosity, 

 and the prominence of the umbonal slope. The relations of this form with 

 MoDiOMORPHA are not satisfactorily determined. 



Formation and locality. In the upper member of the Upper Helderberg lime- 

 stone, at Clarence Hollow, Erie county, N. Y. 



MODIOMORPHA COMPLANATA, 



PLATE XXXIV, FIG. 14 ; AND PLATE XLI, FIG. 3. 



UodloiMirpha coiaplaiMta, Hall. Pi-elim. Notice Ijamellibranchiata, 2, p. 73. 1870. 



" " Pal. N. Y., vol. V, pt. 1. PlaU's ami Explanations : PI. 34, fig. 14 ; pi. 38, 



tiffH. 1-16. 1883. 

 " pennxUa, Hall. In eiTor. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 41, fig. 3, 



1883. 



Shell large, broadly sub-ovate; length one-third greater than the height; basal 

 margin very broadly rounded; posterior margin abruptly and regularly 

 rounded ; cardinal margin arcuate. Anterior end narrow, sub-truncate just 

 below the beak and abruptly rounded below. 



Valves moderately convex ; greatest convexity anterior to the middle. 



