LAMELLIBRANCHIATA. 285 



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. 

 mytiloides. 



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. 

 37 



