452 DESCKIPTIONS OF NEW SPECIES OP FOSSILS FKOM THE 



Beaks from one-third to three-fifths of the leno^th of the shell from the anterior 

 end, extending above the hinge line, compressed, sharp, not prominent, 



Umbonal ridge distinctly defined, subangnlar, with a shallow depression below 

 it, which is most conspicuous at the postbasal margin, becoming obsolete on the 

 upi^er half of the shell ; posterior slope marked by fine, sharp, slightly divergent 

 strife. There are also fine concentric striations, which on the specimens observed are 

 obscure, the surface appearing smooth. 



Hinge line marked by frequent ci-enulations. 



Three specimens measured have each a length of 10 mm. ; height varying from 

 5 to 7 mm. 



In the striation of the posterior slope this species resembles Nucula poststrtata, 

 of the Trenton and Hudson River groups, but may be distinguished from that species 

 by its less gibbous form, the constriction of the postbasal margin, and the conspicu- 

 ous depression below the umbonal ridge. 



Information mid locality. Clinton group, McKee's ore bank, seven miles north- 

 west of Lewistown, Mifflin county, Pennsylvania. 



KUCULA SUBTRIGONA Simpson, n. sp., Fig. 20. 



Shell somewhat variable in form, usually subtrigonal ; length and height about 

 equal ; basal margin i-egiilarly rounded, not constricted toward the posterior end ; 

 viii| ,os^'^2 -rt<^^ 2 posterior margin rounded or obscurely truncate; 



i^ *^ ■ anterior margin abruptly rounded ; cardinal line 



very abruptly declining anterior to the beak, more 

 A.p.s. Trio;;a. gi'ailually declining to the posterior. 



Valves slightly convex, somewhat flattened as they approach the basal margin. 



Beaks about one-third the length of the shell from the anteiior end, not promi- 

 nent, compressed, extending above the hinge line ; umbonal I'idge obscure, very 

 slightly arching upward ; posterior slope very narrow, rounded. 



Surface marked by fine concentric stria? and occasional varices of growth. The 

 concentric 8tri:e are often very obscure, the shell appearing essentially smooth. 



Hinge line, postei-ior to the beak, marked by a row of fine transverse teeth. 



Three specimens representing the extremes in form measure respectively 10, 4 

 and 4 mm. in length, and 10, 3 and 2 mm. in height. Larger specimens occur. 



This species may easily be distinj^mishcd from iV. sinuosa by the absence of a 

 constriction in the basal margin, furrow below the umbonal ridge and striie on the pos- 

 terior slope. 



