10 



angular foramen ; surface marked by from twelve to sixteen 

 angular plications, which often terminate abruptly about 

 one-third of the distance from base to beak, sometimes be- 

 coming obsolete on the upper half of the shell. 



Length, '11, width *10 of an inch. 



Locality. — Spergen Hill. 



Rhyxconella Grosvenom. — Shell globose, or sub-triangu- 

 lar, rotund or depressed; ventral (dorsal) valve more convex 

 than the other, greatest convexity of the two valves near 

 the front, sloping abruptly towards the beak where the two 

 sides meet at nearly a right angle ; beak rather small, neatly 

 defined, nearly straight or slightly incurved, with a linear 

 or sub-triangular foramen j beak of opposite valve round 

 and obtuse, closely incurved. Surface marked by from 

 fourteen to eighteen distinct rounded, simple plications, 

 which often become obsolete towards the beaks ; four or 

 five of the folds depressed, forming a sinus on the larger 

 valve, with a corresponding elevation of five or six plica- 

 tions on the opposite valve. 



Length, '14 to '22, width '13 to '23 of an inch. 



Locality. — Spergen Hill ; Bloomington, la. ; Alton, HI. 



i Ehyncoxella mutata — Shell sub-trigonal, more or less 

 gibbous, front broadly rounded or nearly straight, abruptly 

 tapering to the apex, the two sides meeting at an angle of 

 nearly 90° ; ventral (dorsal) valve much more convex than 

 the opposite one, which is often depressed, shell most con- 

 vex near the anterior margin ; beak of dorsal valve nearly 

 straight or but slightly incurved ; foramen triangular ; beak 

 of the opposite valve obtusely angular and closely incurved 

 against the dorsal valve. Surface marked by from twelve 

 to sixteen strong, sub-angular plications, about four or five 

 of w T hich are depressed in the sinus of the dorsal valve ; 

 sinus not deeply impressed on the margin of the shell ; con- 

 centric stria? rarely visible. 



Length, *15 to *30, width *14 to '32 of an inch. 



Locality. — Above Alton, Illinois. 



