292 PALAEONTOLOGY OF NEW YORK. 



become lamellose. Anterior muscular impression shallow, situated just 

 within the anterior margin. 



The specimen described has a lengtlx of 50 mm., and a height of 27 mm. 



This species bears some resemblance to M. mytiloides, but the beak and 

 umbonal region are much less prominent, the anterior end is broader and the 

 bo<ly of the shell does not expand so rapidly toward the posterior end, mak- 

 ing the hinge-line less oblique. 



Formation and locality. In the Chemung group, Mansfield, Tioga county. Pa. 



MODIOMORPHA HYALEA. 

 PLATE XLI, FIGS. 28-30. 



ilod'unmtrylia hyalea. Hall. Pi-elim. Notice Lainellibranchiata, 2, yi. 79. 1870. 



Pal. N. Y., vol. V, pt. 1. Plates ami Explanations : PI. 41, figs. 28-30. 1883. 

 " J " " Pal. N. y., vol. V, pt. 1. Plates and Explanations : PI. 40, figs. 5-11 (errone- 



ous i-eference). 1883. 



Shell larger than the medium size, sub-elliptical; length nearly twice the 

 height ; basal margin nearly straight, or slightly arcuate near the middle, 

 recurving somewhat abruptly at the extremities. Posterior end abruptly 

 rounded, obliquely sub-truncate above, and curving into the cardinal line. 

 Cardinal margin nearly straight, and parallel to the basal margin of the 

 shell. Anterior end gently declining from the beaks, produced, and abruptly 

 rounded below ; not limited. 



Valves moderately convex in the lower and posterior part, becoming more 

 convex and scarcely gibbous in the umbonal region. 



Hinge-line straight, less than half the length of the shell. 



Beaks broad, moderately prominent, curving over the hinge-line, situated 

 a little less than one-third the length from the anterior end. Umbonal 

 region prominent, scarcely gibbous ; the ridge prominent and rounded but 

 not gibbous. 



Test thin, marked by closely arranged concentric striae, with occasional 

 stronger undulations which are more frequent on the umbonal region. Mus- 

 cular impressions unknown. Surface of the cast for some distance within 



