NATURAL SCIENCES OF PHILADELPHIA. 329 



flat for a half or two-thirds of its length, and is then bent upwards at various 

 angles, often nearly a right angle. The line at which the two valves meet 

 is well defined. Hinge short, rounded, triangular, not very marked. 



The shell is usually about an inch long. I have never seen one more than 

 an inch and a half in length. Prof. Safford, State Geologist of Tennessee, 

 sent me numerous species, from the " Ripley Group " of that State. 



I shall name the Eocene species, in honor of Dr. Morton, 



0. Mortonii G., 0. panda M. pars. Syn. pi. 10, fig. 10. 



Shell oblique, triangular to subquadrate. Both valves strongly plicated. 

 Lower valve, especially in young specimens, usually convex, deepest in the 

 middle, but rarely bent abruptly, and then usually from having been encrust- 

 ing. Hinge distinct, very oblique, and nearly as high as wide, with a distinct, 

 medial depression. Placing the lower valve in its natural position, with the 

 beak furthest off, the left basal angle is always most produced, the right 

 side from the hinge to this angle being more or less regularly curved, and the 

 left side nearly or quite straight. Surface marked by a slightly variable 

 number of ribs, usually about eleven, one or two of which are often much 

 larger than the others ; the longest side being without ribs, except in very 

 large specimens, when there will sometimes be found a dozen small supple- 

 mentary ones. Upper valve nearly or quite flat, and always plicated as 

 strongly as the lower ones, and in a corresponding manner. As the shell 

 increases in size, the form and markings become less regular, sometimes the 

 plications equalling in depth one-third of the length of the shell. 



Dr. Morton's figure is a very accurate representative of the original, now 

 before me. Another specimen from the same bed is 2-5 in. long. Width, 3 in. 

 Diameter, 1-5 in. Greatest oblique width, 3-5 in. 



Very common in the Eocene limestone of South Carolina, and in the same 

 formation from Alabama. 



Ostrca Vicksburgensis Cuv., can be distinguished from this species by its 

 more regular form, deeper valves, and smooth, rounded ribs. 



GRYPHiEA Lam. 



G. Thirsae. Rounded-subtriangular. Lower valve; beak very small, 

 and close to the hinge, never exsert. Umbone rounded, very prominent and 

 somewhat compressed laterally, the rounded elevation continuing more or 

 less regularly, becoming broader, to the middle of the basal margin, 

 at which point this margin is always somewhat emarginate. Ligament 

 area broad, triangular, transversely striate, and with a slight, irregular de- 

 pression in the middle. Interior of valve very deep. Muscular impression 

 nearly ovoid, narrowest on the inner end. External surface marked by a 

 few small, irregular squamose ridges, most numerous and distinct directly 

 behind the emargination of the base. Upper valve unknown. 



The species resembles, remotely, some of the narrower forms of G. vesicu- 

 laris Lam., but after comparing the series before me with numerous authentic 

 specimens of that species, both American and European, some of the latter 

 labelled by d'Orbigny and others, by Charlesworth, I am satisfied that they 

 are distinct. The beak is so small as to be almost obsolete, and there is 

 always a more or less distinct, rounded, umbonal ridge. In general form, it 

 resembles G. (Exogyra) columba, but wants the spiral beak, and is never 

 lobed. The small beak and absence of all traces of lobes will sufficiently 

 separate it from G. Pitehcrii. 



The specimens are in the Museum of the Smithsonian Institute (No. 570 , l, 

 and are from a light gray sandy marl. 



Locality, " Nanafalia," Alabama. 



Length, 1.7 in. Greatest width, 1-3 in. Width at the hinge, -6 in. Greatest 

 height of valve, -8 in. Height at the hinge, -7 in. 'Length from the basal 

 margin, over the umbone, to the beak, 2-3 in. 



1861.] 22 



