11 



C Rhyxcoxella subcuneata. — Triangular, subcuneate ; 

 front rounded, meeting the lateral slopes at an obtuse angle ; 

 sides sloping to the beak and meeting at an angle of 

 60° or 65° j valves nearly equally convex, dorsal valve most 

 convex towards the beak ; beak of dorsal valve very acute, 

 scarcely incurved, and perforate by a triangular foramen ; 

 beak of ventral valve acute, closely incurved below the tri- 

 angular foramen. Surface marked by about twelve to four- 

 teen, (and rarely sixteen,) strong, simple, angular plications, 

 which are somewhat obsolete near the beak ; scarcely any 

 indication of a sinus ; plications crossed by fine concentric 

 stria?, and in old shells, at irregular distances, by stronger 

 imbricating folds or wrinkles parallel to the lines of growth ; 

 sides of both valves beneath the beak free from plications, 

 and forming a very distinct elongate-oval space. 



Length, '16 to *41, width *15 to *39 of an inch. 



Locality. — Bloomington ; Spergen Hill. 



Rhyxcoxella macra. — Shell triangular, flattened; apex 

 acute ; valves nearly equal ; the dorsal valve a little more con- 

 vex towards the beak, which is quite straight, extended be- 

 yond the lesser valve, and with a sub-triangular foramen, 

 which is slightly rounded above. Surface marked by from 

 eighteen to twenty-four small, rounded plications which are 

 about equal to the spaces between. 



Length, 'lb to '24, width *14 to '29 of an inch. 



Locality. — Alton, 111. 



--^Rhyxcoxella Worthexl— Shell small, longitudinally sub- 

 trigonal, very abruptly tapering to the apex ; ventral valve 

 very convex or gibbous towards the front ; dorsal valve 

 nearly flat and broadly sinuate in front, with a single broad, 

 flattened plication, commencing near the margin, and filling 

 a deep sinus in the opposite valve, corresponding to two 

 short rounded plications on the front of the ventral valve ; 

 edges of the shell on each side of the mesial sinus sharply 

 undulated, with indistinct marginal folds ; beak of dorsal 



