LAMELLIBRANCHIA TA. 
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abruptly both at the anterior and posterior extremities; posterior margin 
abruptly curving below and more gently forward to the cardinal margin; 
cardinal margin arcuate from the beaks to the post-cardinal extremity. 
Anterior end short, scarcely defined, narrow, declining from the hinge-line 
and regularly rounded below. 
Valves convex, very gibbous in the middle and umbonal region; depth 
more than half the height of the shell. The umbonal elevation extends 
from the beaks, curving above the middle, to near the post-basal margin, 
forming an undefined arcuate ridge. 
Hinge-line oblique, extending more than one-third the length of the valves. 
Beaks small, sub-anterior, closely incurved. 
Test comparatively thick, marked by fine concentric striae, which are fas¬ 
ciculate and raised into strong abrupt ridges upon the umbonal and anterior 
portions of the shell. Interior unknown. 
The principal specimen described has a length of 70 mm., height 41 mm., 
and a depth in both Valves of 29 mm. A smaller specimen referred with 
some doubt to this species has a length of 50 mm., height 32 mm., depth 21 
mm. This specimen shows a strong anterior muscular impression with an 
accessory pit, and also a shallow posterior muscular scar on the post-cardi¬ 
nal slope. 
This form resembles M. alta , but is more elongate, distinctly arcuate; the 
anterior end is narrower below the beaks; the posterior end is more produced 
and curving more abruptly forward in the upper part, with the post-cardinal 
angle rounded. The anterior end is less produced than in typical forms of M. 
mjtiloides. 
Formation and localities. The original specimen is recorded as coming from a 
compact limestone in the Hamilton group of Onondaga county, N. Y. The 
small specimen referred to is from the Hamilton group in Clarke county, Ind. 
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