LAMELLIBRANCHIATA. 289 



Valves moderately convex, except in the umbonal region, where they are 

 somewhat gibbous in the middle. 



Hinge-line slightly arcuate, extending about half the length of the shell. 



Beaks small, sub-anterior, incurved. Umbonal region prominent, gradually 

 declining to the post-cardinal slope. 



Test thin, marked by fine concentric striae. Interior unknown. 



The specimen described is somewhat distorted by pressure, and has a length 

 of 50 mm., and a height of 25 mm. 



In its normal condition this species would resemble M. subalata and its vari- 

 ety, and may be compared with figs. 5, 10 and 11 of plate xli. The shell is 

 nearly all exfoliated, and the surface characters are thereby rendered obscure. 



Formation and locality. In the arenaceous shales of the Chemung group, at 

 Belmont, Alleghany county, N. Y. 



MODIOMOKPHA QUADRULA. 



PLATE XLI, FIGS. 18-26. 



Mod'unmtrpha quadrula. Hall. Prelim. Notice Lamellibraiichiata, 2, p. 77. 1870. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 41, figs. lS-26. 



1883. 



Shell small, quadrangular or rhoinboidal, varying in length and height from 

 nearly equal to almost twice as long as wide ; basal margin nearly straight 

 or slightly convex, curving abruptly into the anterior and posterior margins. 

 Posterior extremity sub-truncate, sometimes nearly at right angles to the 

 ba.sal line. Cardinal margin nearly straight or gently curved. Anterior 

 end declining abruptly from the beaks, narrow and abruptly rounded, not 

 limited by any defined sinus. 



Valves moderately convex toward the basal and posterior margins, slightly 

 gibbous in the umbonal region. 



Hinge-line scarcely oblique, extending two-thirds the length of the shell. 



Beaks sub-anterior, small and closely appressed. Umbo rising above the 

 hinge-line and extending in a more or less defined obtusely angular ridge to 

 the post-basal extremity. 



