APPENDIX. 11 
and latter species described not only in the above mentioned character, but 
in having the umbo directed obliquely towards the posterior end. The 
only exception I know of is a species figured by Deshayes. So slight a 
variation from the type of a genus, as the spiral beaks and peculiar 
obliquity, when it marks a group of species peculiar to a formation, has a 
significance which entitles it to notice, and especially when one believes 
that all life was destroyed at the close of the Cretaceous era. In all the 
Eocene species the smaller valve has involute and concealed beaks, and in 
all the later fossil and recent species this character is preserved, except in 
the Tertiary genus Paciiydon, Gabb, which name must be retained to 
designate Mr. Gabb’s genus, as Paciiyodon, Stuchbury is superseded by 
Thallassides, Bergner. In this group the umbo when oblique points 
towards the anterior end or in the opposite direction to that of the Cre- 
taceous group. Corbula obliquata Desh. is a small shell which has 
a very slight obliquity, exaggerated in the enlarged figure, if the small 
outline is correct. 
C. Carolinensis , pi. 2, fig. 15. Shell oblique ; right valve ventricose, the 
left inflated, concentrically striated, posterior side very short, suddenly 
contracted ; end of left valve obliquely truncated, reflexed, submargin 
minutely carinated. 
Common, generally with separated valves, not water-worn. I found 
one specimen with connected valves. 
C. perbrevis , pi. 2, fig. 5. Shell elevated, with a short, truncated wing 
and concentrically marked with prominent strife ; posterior submargin 
carinated from beak to posterior extremity ; summit prominent. 
C. subgibbosa , pi. 2, fig. 12. Shell triangular, equilateral, rostrated, 
obliquely truncated at the posterior end ; disk with concentric obsolete 
lines; ventral margin gibbous; valves carinated from beak to posterior 
extremity, submargin deeply impressed. 
C. oxynema. Shell ovate acute, inequilateral, ventricose ; posterior side 
cuneiform, compressed ; disk delicately sculptured with fine, close, sharp 
equal lines. 
Locality: Cape Fear river. 
C. bisulcata, pi. 2, fig. 13, 11. Shell ovate-acute, equilateral, concen- 
trically striated, with two or three distant large concentric furrows; pos- 
terior extremity acute. 
