PLATE IV f. 
Legend. F. Foramen. 
p. Pedicle-area. 
p / . Areal callosity. 
g. Base of the external pedicle-groove. 
g'. Base of the internal pedicle-groove. 
1. Lateral walls of the groove. 
ar. Anterior muscular ridges, 
s. Median septum, 
v. Vascular sinuses. 
DISCINISCA, 
Dall. 
Page 120. 
Dlscinisioa lamellosa. Broderip. 
Fig - . 1. The internal pedicle-area, enlarged to three diameters from the specimen represented in Plate 
IV e, fig. 2. The pedicle-area is a somewhat thickened elevation, corresponding to the deep 
depression on the external surface of the valve. The foramen transects the area directly, and 
between it and the posterior margin is a narrow translucent lamina, across which the concentric 
ornamentation of the valve does not pass. 
SCHIZOTRETA, Kutorga. 
Page 120. 
OBICULOIDEA (SCHIZOTRETA ?) TENUILAMELLATA, Hall. 
Fig. 2. The exterior pedicle-area of a specimen in which the groove is extremely short. X 5. 
Fig. 3. A similar view of a specimen in which the groove is relatively larger, and the base, walls and 
foramen well defined. X 5. 
Fig. 4. The corresponding parts in another specimen. X 5. 
Fig. 5. The groove of the same ; showing that the concentric growth-lines are retained on the broad lat¬ 
eral walls, while the base is smooth though less distinctly defined than in some instances. X 16. 
Fig. 6. The pedicle-characters of the interior, drawn from a natural cast of the exterior, in which, from 
the compression of the shell, the internal characters are preserved in continuity with those of 
the outside. 
Niagara limestone. Hamilton, Ontario. 
SCHIZOTRETA CONICA, Dwight. 
The external characters of the pedicle-area ; from the specimen represented in Plate IV e, fig. 7. 
X 3. ° 
ORBICULOIDEA, D’OrbigxNt. 
Page 120. 
Orbiculoidea PULCHRA, sp. 110V. 
The internal pedicle-area. The elevated median ridge corresponding to the base of the external 
groove, is slightly broken, a crack extending to the foramen. About the anterior moiety of 
the ridge the lateral callosities have just begun to form. X 3. 
The same, in a more advanced stage of development. The callosities have increased to such a 
degree as to have almost come into contact for the entire length of the area, leaving the posi¬ 
tion of the internal ridge indicated by the groove between their edg'es. X 3. 
Cuyahoga shales. Berea, Ohio. 
The same parts, still showing a remnant of the groove formed by the meeting of the lateral cal¬ 
losities ; also indicating the position of the foraminal opening. X 3. 
Cuyahoga shales. Baconsberg, Ohio. 
Figs. 11, 12: The same features, from individuals in which the lateral callosities have coalesced, leaving the 
foramen open, but otherwise concealing the structure of the area. X 3. 
Fig. 13. The interior of the apical portion of the brachial valve; showing the small median septum, extend¬ 
ing forward from the apex. X 3. 
Cuyahoga shales. Befrea, Ohio. 
Fig. 30. An internal cast of a brachial valve ; showing the impressions of two strong, diverging vascular 
sinuses (?) and finer markings about the margin. X 1.5. 
Cuyahoga shales. Newark, Ohio. 
Fig. 32. A very young pedicle-valve, with a length of 2.25 mm., having the posterior margins of the 
aperture unclosed, while the pedicle-groove has nearly its entire normal length. X 12. 
Cuyahoga shales. Berea, Ohio. 
Fig. *7. 
Fig. 8. 
Fig. 9. 
Fig. l(b 
Orbiculoidea, compare O. pulchra. 
Figs. 14, 15, 16. The internal characters of the pedicle-area; showing the envelopment of the ridge by the 
lateral callosities, in various stages of advancement. X 3. 
Waverly group. Meadville, Pa. 
Orbiculoidea marginalis, Whitfield. 
Fig. 17. The internal characters of the pedicle-area. The callosities have assumed an irregular, but bi¬ 
laterally symmetrical form, almost enveloping the pedicle-ridge. X 3 
Hamilton group. Cementvillr, Wifi. 
