272 
PALAEONTOLOGY 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 umbonal 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 
both 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. 
Modiomorpha complanata, Hall. Prelim. Notice Lamellibranchiata, 2, p. 73. 1870. 
“ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 34, fig-. 14 ; pi. 3S, 
figs. 1-16. 1883. 
“ peruvata, Hall. In error. 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. 
