LAMELLIBRANCHIA TA. 
261 
margin abruptly rounded, gently curving into the dorsal margin which is 
straight above the middle. 
Right valve gently convex in the posterior part, becoming gibbous and 
angular in the umbonal region. Left valve unknown. 
Beak erect, acute, pointed. 
Test of moderate thickness, marked by fine concentric striae of growth, 
which at intervals have been raised into lamellae which have left strong 
varices upon the cast. 
Interior unknown. 
A right valve has a length of 65 mm. and height 24.5 mm. 
This species differs from M. Chemungensis in its more elongate, straighter form, 
and in the angularity of the umbonal slope. From M. carinata it is distin¬ 
guished by its elongate form, straight ventral side, and more prominent anterior 
umbonal slope. 
Formation and locality. In the arenaceous beds associated with fossils of the 
middle Chemung group, from a loose mass collected at Elmira, N. Y. 
Mytilarca simplex. 
PLATE XXXIII, FIGS. 19, 21. 
Mytilops ( Modiola ) simplex. Hall. Pal. 1ST. Y., vol. v, pt. 1. Plates and Explanations: PI. 33, figs. 19, 
21 (20 in error) Jan., 1883. 
• Shell of medium size; body elongate-ovate, with an acute apex; length nearly 
twice the height; byssal area depressed; margin below nearly straight for 
two-thirds the length; posterior margin abruptly rounded; dorsal margin 
gently curving to the hinge-line. 
Yalves equal, moderately convex in the posterior part, and gibbous on the 
umbo. The line"of greatest convexity is on the ventral side of the axis. 
Hinge-line short. Beaks sub-erect, acute, abruptly attenuate. 
Test marked by fine, closely arranged, concentric strise. 
Ligamental area and interior unknown. 
A specimen has a length of 41 mm. and height 25 mm. 
