26 
APPENDIX. 
REMARKS OjST SOME GENERA OF SHELLS, 
T. A. CONKAD. 
PROTOCARDIA. Bey rich. 
The type of this genus is P. II Ulan a, Sowerby, a cretaceous fossil, and 
strictly defined, the genus first appears in the Lias, in one species, P. Phil- 
lipsii, Conard ( P . striatula , Phillips, not Sowerby.) The Carboniferous 
allied forms are Cardium dissimilis and C. striatulurn, Sowerby, which 
I exclude from the genus. 
Pkotocakdia proper consists of ventrieose shells which are seuptured 
with concentric lines ending at the angle ot the umbonal slope, and having- 
entire radiating ribs on the post-utnbona! slope ; umbonal slope slightly 
incurved. 
The above diagnosis applies only to the Cretaceous species and excludes 
Cardium bisectum , Forbes, which Stlioczka includes in the group. 
Eocene Group. 
Shell having fine radiating lines ; post umbonal slope with acute tuber- 
cles on the ribs. 
The aberrant forms are P. difficile , Deshayes, P. Plumsteadiana , 
Sowerby, and P. nitens , Sowerby, which have smooth posterior ribs, but 
radiating lines on the disk. 
Older Oligocene Group. ( Jackson Group.) 
Agree in scupture with the Eocene species. 
Newer Oligocene Group. ( Vicksburg Group.) 
Decussated strike ; entire radiating lines on the post-umbonal slope. 
Recent. 
Decussated striae ; short spines on the post-umbonal slope. 
