PLATE IV F. 



Lefreini. F. Fdramen. 



ji. Pedicle-area, 

 p'. Areal (-allosity. 



g. Base of the external pedicle-g-roove. 

 g'. Base of the internal pedicle-gTOOve. 



1. LaliM-al walls of the groove, 

 ar. Anterior muscular ridges. 

 s. Median septum. 

 V. Vascular sinuses. 



Fig. 



Fig. 

 Fig. 



Fig. 

 Fig. 



Fig. 



Fig, 



DISCINI8CA, Dall. 



Page 120. 

 DiSClNISIC.V LAJIELLOSA, IJlodt'lip. 



1. The internal pedicle-area, enlarged to three diameters from the specimen repi-esented in Plate 



IV E, tig. 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 ami the posterior margin is a narrow translucent lamina, across which the concenti-ic 

 ornamentation of the valve does not piass. 



SCHIZOTRETA, Kutorga. 



Page 1-20. 

 ObICULOIDEA (ScHIZOTRETA ?) TENUILAMELLATA, Hilll. 



2. The exterior pedicle-area of a specimen in which the groove is extremely short. X 5. 



3. A similar view of a specimen in which the groove is rela.ti\ely larger, and the base, walls and 



foramen well defined. X 3. 



4. The corresponding parts in another specimen. X S. 



5. The groove of the sauie ; showing that the concentric growlli-lines are retained on the broad lat- 



ei'al walls, while the base is smooth though less distinctly detined than in some instances. X Ifi. 



6. The pedicle-characters of the interioi-, drawn from a natural cast of the exterior, in which, fi-om 



the compression of the shell, the internal chai'acters are preserved in continuity with those of 

 the outside. 



Niagara limestone. Hamilton, Ontario. 



ScHIZOTRETA CONICA, Dwight. 



from the specimen represented in Plate IV e, tig. 7. 



D'Orbig.nt. 



The external characters of the pedicle-area ; 

 X 3 



ORBICULOIDEA, 



Page 1-20. 



Orbiguloidea pulciira, .sp. nov. 

 Fig.' 8. The internal pedicle-area. The elevjited median ridge cori-espouding to the base of the external 

 groove, is slightly bi'oken, a crack extending to the foramen. About the antei'ior moiety of 

 the ridge the hiteral c;dlosities have just liegun to form. X 3. 

 Fig. 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 le:igth of the area, lea^-ing the posi- 

 tion of the internal ridge indicated by the gi'oove between their edges. X 3. 

 Cuyahoga shales. Berea, Ohio. 

 Fig. 10. 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. Bacon.fberg, Ohio. 

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



foramen open, but otherwise concealing the structurij of the area. X 3. 

 Fig. 13. The interioi' of the ajiical portion of the brachial valve : showing the small median septum, extend- 

 ing forward from the apex. X 3. 



Cuyahoga shales. Berea. Ohio. 

 Fig. 30. An intei-nal c;ist of a biachial valve ; showing the impressions of two strong, diverging vascular 

 sinuses (1) and finer markings about the margin. X l-o. 

 Cuyahoga shales. Neimrk, Ohio. 

 Fig. 32. A very young pedicle-valve, with a length of 2.2.5 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. 



Orbiguloidea, compare O. pulchra. 



Figs. 14, 15, 16. The intei'nal ch.aracters of the pedicle-area ; showing the envelojiment of the ridge by the 

 latei'al callosities, in various stages of advancement. X 3. 

 Waverly grouyi. Meadxnlle, Pa. 



Orbiguloioea .margi.n'alis, Wliilticld. 



Fig. 17. The internal chatacters of the pedicle-area. The callosities have assumed .an irregular, t>ut bi- 

 laterally symmetrical form, almost enveloping the pe<licle-ridge. X 3 

 Hnniilton gi-ou]>. CtntrnfriUr. fF/.?. 



