Weller — Kinderhook Fanned Studies. 169 



mm., and a maximum width near the posterior extremity of 

 4 mm ; subcuneate in outline, anterior extremity sharply 

 and narrowly rounded ; posterior extremity broadly rounded, 

 dorsal margin straight, ventral margin slightly convex; the 

 beaks situated above the anterior extremity; l he hinge-line 

 nearly equalling the extreme length of the shell. Extending 

 from the beak to the posterior basal extremity is a well- 

 developed umbonal ridge which is rather sharply rounded 

 anteriorly but becomes more broadly rounded posteriorly ; 

 the dorsal slope from the umbonal ridge is nearly vertical and 

 slightly concave near the beak, but becomes less abrupt and 

 slightly convex posteriorly ; the ventral slope is convex 

 throughout ; the greatest convexity of the shell is upon the 

 umbonal ridge about one-third the length of the shell from 

 the beak. 



Remarks. This little shell is apparently cogeneric with 

 several species in the American Mississippian faunas, which 

 have usually been referred to the genus Lithophaga. It is 

 quite possible that these shells are not really members of 

 Lamarck's genus, but they may be retained here for the 

 present for the want of a better place for them. DeKoninck 

 has referred several quite similar species from the Carbonif- 

 erous fauna of Belgium, to the genus Modiola. 



IVUCKODON PARVUS W. & W. 



PL XV. f. 14. 



Original description. " Shell small, elongate quadrangular, 

 or area-form ; length equal to twice and a half the breadth. 

 Valves extremely ventricose. Beaks prominent and incurved, 

 situated at about two-fifths of the entire length from the an- 

 terior end. Hinge line straight, nearly as long as the body 

 of the shell. Posterior end obliquely truncate, somewhat 

 prolonged at the postero-basal angle. Anterior end gradually 

 rounding from the hinge on to the basal margin, which is 

 gently arcuate, with a slight emargination in the middle, 

 forming a small byssal opening. Hinge plate narrow, bear- 

 ing on the posterior end two long linear, lateral teeth ; the 

 inner one the longest, reaching nearly one-third the length of 

 the shell ; the anterior end having about four short, oblique 



