214 Eocene Fossils of the United States. 
Pholas petrosa, Con. ; Proceed. Nat. Inst., p. 193, Plate II, fig. 4. 
Piscataway, Prince George’s Co., 
Occurs in an indurated marl at the sumnitiait of the Eocene, and 
is rare and imperfect ; econ y | in casts, witha trace of the shell 
remaining. ie ee a Dh 
PHoLADOMYA. ae 
I believe this interesting genus first appears in the Oslite group. 
Several species from rocks of that period have been described by 
Sowerby. M. Marcel de Serres described a species in the ter- 
tiaty marls of the south of France, and one recent species has 
been figured and described by Sowerby in his “ Genera of Shells.” 
This inhabits the West Indies. In this country the oldest species 
we find is from the cretaceous strata. 'T'wo tertiary species have 
been described by myself; one belonging to the Eocene, the other 
to the Miocene period. 'The latter has been erroneously referred 
to Panopea by Deshayes. From that genus it widely differs, 
in having a pearly nacre and in being destitute of cardinal teeth. 
The young or middle aged shells are extremely fragile, and have 
scarcely a trace of the muscular or palleal impressions, resembling 
in this respect, the recent species, with which the hinge also cor- 
responds. 
Puotapomya Marytanpica. Ovate, profoundly ventricose, with 
coarse, irregular concentric lines and furrows, and obsolete, rather 
indistinct radii ; umbo summit prominent; anterior side very short, 
the extremity subtruheated ; basal = rounded ; posterior ex- 
tremity rounded. (Plate I, fe 9.) 
Pholadomya Marylandica, Con. ; Proceed. Nat. Inst., p. 193, 
Plate I, fig. 3. 
Pissinway, Md. 
This rare shell accompanies Pholas petrosa, and is in the same 
state of preservation. Little of the shell remains, and that exhibits 
the pearliness which no doubt characterized all the species, and 
which prominently distinguishes this genus from Panopza. 
Panopma. 
‘The genus Panopxa, of Menard de la Groye, embraces few 
recent species. Sowerby has described one from the Great Oolite 
and another which is supposed to occur in the Green Sand forma- 
tion. Deshayes enumerates three fossil species, one in the Eocene 
of Paris, the other in Miocene deposites. In this country, I find 
