PLATE IV c. 



Legend. a. Deltidium. 



b. Deltidial slope. 



c. Deltidial ridges, 

 ca. Cardinal area, 

 eg. Cardinal groove, 

 ci'. Crescent. 



d. Areal borders. 



h. Cardinal buttress, 

 i. Umbonal chambers. 



n. 

 o. 



q- 



r. 



u. 

 iV. 



Platform. 

 Platform-vaults. 

 Median scars. 

 Anterior scars. 

 Lateral scars. 

 Crown of ci'escent. 

 Side of crescent. 

 Pallial trunks. 

 Secondary pallial sinuses. 



Fig- 

 Fig. 



MONOMERELLA, Billings. 



Page 40. 



MoNOMEEELLA Newberryi, Hiill and Wliitliekl. 



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 anil the deep cardinal 



groove. 



Niagara limestone. Q-enoa, Ohio. 



MONOMEUELLA (cnf. OKBICULAKIS, BlUillgs). 



Fig. 3. Profile view of an internal cast. 



Fig. 4. Another view of the same specimen; showing the internal featui'es of the pedicJe-valve. . The 

 cardinal groove and the crescent are very conspicuous, the umlional chambers short, scarcely 

 reaching the hinge, and the lateral muscular impi-essions very sharply developed. 



Fig. 5. The opposite side of the same specimen ; showing- the low umbonal chambei-, the strong crescent, 

 and incipient platform-vaults. 



Niagara limestone. Between (Grafton and Cedarhurg, Wisconmi. 



MONOMERELLA PRISCA, BiUillgS. 



Pig. 6. An internal cast of a portion of the pedicle-valve, in which the cardinal area is vei-y high and the 



platform slightly unsymmetrical. 

 Fig. 7. The interior of a peilicle-valve, drawn from a gutta-percha impression of the specimen repi-esented 



in fig. 6, and slightly restored about the antei-ior portion. 

 Fig. 8. The intei-ioi' of a pedicle-valve, in which the deltidium is narrow and elevated, the deltidial ridges 



obscure, and the ai-eal boi-ders smooth. From a gutta-pei-cha impression of a natu;-al cast. 

 Fig. 9. An internal cast of a pedicle-valve, in which the umbonal chambers have their usual length. 



The ci-escent and lateral scars of the platform are unusually distinct. 

 Fig. 10. An internal cast of the pedicle-valve, in which the umbonal chambei's ai-e of unequal length. 

 Fig. II. An internal cast of both valves, to which a portion of the matrix adhei-es ; showing the thickness 



of the shell in the umbonal regions. 

 Fig. 12. An internal cast of a brachial valve, with indistinct i-adiating, probably vasculai' furrows in the 



anterioi- I'egion. 

 Fig. 13. An intei'nal cast of a brachial valve ; showing- the chai-acteristic markings of the shell. 

 Niagara limestone. Hawthorne, Illinois. 



MONOMERELLA OrTONI. Sp. IIOV. 



Fig. 14. An internal cast of the pedicle-valve, in which all the charactei-s of the shell are very sharply 



defined. 

 Fig. 15. The counterpart of the same valve, from a gutta-percha impression ; showing the broad deltidium 



without evidence of subdivision, the conspicuous cai-dinrd slope and groove, the crescent and 



platform scars and the pallial trunks with their ramifications. 

 Niagara limestone. Rising Sun, Ohio. 



