Weller — Kinderliook Faunal Studies. 453 



which is convex above aud rather abruptly rounded below into 

 the nearly straight or slightly concave ventral margin, the 

 anterior margin short and abruptly rounded, produced some- 

 what beyond the beaks in -the younger specimens but nearly 

 equal with the beaks in older individuals. Valves depressed 

 convex, with an arcuate, more or less subangular umbonal 

 ridge extending from the beak to the postero-ventral margin, 

 becoming less well defined posteriorly; a flattened area or 

 slight sinus extends from the beak obliquely backward across 

 the ventral slope to about the middle of the ventral margin. 

 Surface marked with strong, crowded, concentric lines of 

 growth which become sublamellose toward the margin of the 

 valves. 



The dimensions of three specimens are: length 24, 22, 

 18 mm., width 13.5, 12, 9 mm., convexity 5, 4,3 mm. 



Remarks. This species has its closest relative in Modio- 

 morpha concentrica (Con.), of the Hamilton fauna in New 

 York State. It differs from that species in being more 

 arcuate, in having less regular and more lamellose concentric 

 markings, and in the more anterior position of the beaks. 



MODIOLA MISSOURIENSIS n. sp . 



Plate 2, Jigs. 14-17. 



Description. Shell equivalved, of medium size, obliquely 

 subovate in outline, widest posteriorly, width about two-thirds 

 the length, beaks nearly terminal, small, incurved. Dorsal 

 margin arcuate, curving regularly into the broadly rounded 

 posterior margin, ventral margin slightly convex posteriorly 

 with a shallow byssal sinus anteriorly, in front of which it 

 curves up rather abruptly into the short, subtruncate anterior 

 margin. Valves strongly convex, umbonal ridge well defined 

 towards the beak, merging into the general convexity of the 

 shell posteriorly ; the surface sloping abruptly from the um- 

 bonal ridge to the dorsal margin, much more gently to the 

 ventral margin; an indistinct flattening or slight sinus, some- 

 times nearly or quite obsolete, extends from near the beak 

 obliquely backward across the ventral slope to the ventral 

 margin just back of the byssal sinus. Surface marked with 

 concentric lines of growth which are more frequent towards 



