LAMELLIBRANCHIA TA. 
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. 
Modiomorpha quadrula. 
PLATE XLI, B'IGS. 18-26. 
Modiomorpha quadrula , Hall. Prelim. Notice Lamellibranchiata, 2, p. 77. 1870. 
“ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PL 41, figs. 18-26. 
1883. 
Shell small, quadrangular or rhomboidal, 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 
basal 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. 
