PLATE IV c. 
Fig-. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Legend, a. Deltidium. 
b. Deltidial slope. 
c. Deltidial ridges, 
ca. Cardinal area, 
eg. Cardinal groove, 
cr. Crescent. 
d. Areal borders. 
h. Cardinal buttress. 
i. Umbonal chambers. 
j. Platform. 
k. Platform-vaults. 
m. Median scars. 
n. Anterior scars. 
o. Lateral scars. 
q. Crown of crescent. 
r. Side of crescent, 
u. Pallial trunks. 
uL Secondary pallial sinuses. 
MONOMERELLA, Billings. 
Page 40. 
Monomerella Newberryi, Hall and Whitfield. 
1. An internal cast of the brachial valve, indicating a strong crescent and low umbonal chambers. 
2. The reverse of the same specimen; showing the short umbonal chambers and the deep cardinal 
groove. 
Niagara limestone. Genoa, Ohio. 
Monomerella (enfi orbicularis, Billings). 
3. ( Profile view of an internal cast. 
4. Another view of the same, specimen; shoTjying the internal features pf the, pedicle-valve. The 
cardinal groove and the crescent are ve^y consjpicpous,. the umbonal chambers short, scarcely 
reaching the hinge, and the lateral'muscular'iinpi-eSsions vei-y sharply 1 developed. 
5. The opposite side of the|Same specimen ; showing the low umbonal chamber, the strong crescent, 
and. incipient'plaiffdrrn-Vaults. 
Niagara limestone. Between Gh'af ton and (Jedarfoirg, Tyisconsiu. 
Monomerella prisoa, Billiiigfe. !!:ui 
6. An internal cast of a portion of the pedicle-valve, in which the cardinal area is very high and the 
platform slightly unsymmetrical. 
7. The interior of a pedicle-valve, drawn from a gutta-percha impression of the specimen represented 
in fig. 6, and slightly restored about the anterior portion. 
8. The interior of a pedicle-valve, in which the deltidium is narrow and elevated, the deltidial ridges 
obscure, and the areal borders smooth. From a gutta-percha impression of a natural cast. 
9. An internal cast of a pedicle-valve, in which the umbonal chambers have their usual length. 
The crescent'and lateral scars of the platform are unusually distinct. 
10. An internal cast of the pedicle-valve, in which the umbonal chambers are of unequal length. 
11. An internal cast of both valves, to which a portion of the matrix adheres; showing the thickness 
of the shell in the umbonal regions. 
12. An internal cast of a brachial valve, with indistinct radiating, probably vascular furrows in the 
anterior region. 
13. An internal cast of a brachial valve ; showing the characteristic markings of the shell. 
Niagara limestone. Hawthorne, Illinois. 
Monomerella Ortoni. sp. nov. 
14. An internal cast of the pedicle-valve, in which all tlie characters of the shell are very sharply 
defined. 
15. The counterpart of the same valve, from a gutta-percha impression ; showing the broad deltidium 
without evidence of subdivision, the conspicuous cardinal slope and groove, the crescent and 
platform scars and the pallial trunks with their ramifications. 
Niagara limestone. Rising Sun, Ohio. 
