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 i-iilges. 

 s. Median septum. 

 V. Vascular sinuses. 



DISCINISCA, Dall. 



I^age 120. 



DisciNisic.A LAMELLOSA, Biodeiip. 

 Fig. ]. The internal pedicle-area, enlarged to three diameters from the specimen represented in Plate 

 IV B, fig. 2. The ijedicle-area is a somewhat thickened elevation, corresponding to the deep 

 depression on the external surface of the valve. The foramen ti-ansects the area directly, and 

 between it and the poaterioi' margin is a narrow translucent lamina, across which the concentric 

 ornamentation of the valve does not pass. 



SCHIZOTRETA, Kutorga, 



Page 120. 



Obiculoidea (Schizoteeta ?) 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 

 foi'amen well defined. X 5. 



Fig. 4. The cori'esponding 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 thoug'h 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, fi-om 

 the compression of the shell, the internal characters are preserved in continuity with those of 

 the outside. 



Niagara limestone. Hamilton, Ontario. 



Fig-.- 



Fig. 

 Fig. 



Fig. 



Figs. 

 Fig. 



Fig. 



Fig. 



SCHIZOTRETA CONICA, Dwigllt. 



7. The external characters of the pedicle-area ; from the specimen represented in Plate IV B, fig. 7. 



X 3 



ORBICULOIDEA, D'Orbigny. 



Page 1-20. 



Orbiculoidea pulchra, sp. nov. 



8. The internal pedicle-area. The elevated median ridge cori'esponding 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. 



9. 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 edges. X 3. 

 Cuyahoga shales. Berea, Ohio. 



10. The same parts, still showing a remnant of the groove foi-med by the meeting of the lateral cal- 



losities ; also indicating- the position of the foraininal opening. X 3. 

 Cuyahoga shales. Baco)i.sberg, Ohio. 



11, 12. The same features, from individuals in which the lateral callosities have coalesced, leaving the 



foramen open, but otherwise concealing the sti'uctui';; of the ai'ea. X 3. 

 13. The interior of the apical portion of the brachial valve ; showing the small median septum, extend- 

 ing forward from the apex. X 3. , 

 Cuyahog-a shales. Berea, Ohio. 

 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. Neicark, Ohio. 

 32. A very young pedicle-valve, with a length of 2.25 mm., having the posterior margins of the 



X 12. 



aperture unclosed, while the pedicle-groove has nearly its entire normal length. 

 Cuyahoga shales. Berea, Ohio. 



Orbiculoidea, compiire O. pulchra. 



Figs. 14, 15, 16. The intei'nal characters of the peilicle-area ; showing the envelopment of the ridge by the 

 lateral callosities, in vai'ious stages of advancement. X 3. 

 Waverly grouji. Meadville, Pa. 



OrBICULOIUEA MAEtilNALlS, Wlliltickl. 

 Fig. 17. The internal characters of the pedicle-area. The callosities have as.<uined an irregular, but bi- 

 laterallj' symmetrical foi'm, almost enveloi^ng the pedicle-ridge. X 3 

 Hamilton group. CenirmtlvU". Wi.". 



