PLATE IV c. 



Legeiiil. a. Deltidium. 



b. Deltidial slope. 



c. Deltidial ridges. 

 ca. Cardinal urea. 

 eg. Cardinal groove. 

 cr. Crescent. 



li. Areal borders, 

 h. Cardinal buttress, 

 i. Unibonal chambers. 



j. Platform, 

 k. Platform-vaults, 

 m. Mi^dian scars, 

 n. Anterior scars. 

 0. Lateral scars, 

 q. Crown of crescent. 

 r. Side of crescent. 

 u. Pallial trunks, 

 u'. Secondary pallial sinuses. 



MONOMERELLA, Billings. 



Page 40. 



MoNOMERELLA Nevvbeuryi, Hiill iiiid Whitfield. 



Fig. 1. An internal cast of the brachial valve, indicating- a strong- crescent and low umbonal chambers. 

 Fig. 2. The reverse of the same specinujn ; showing the short umbonal chambers and the deep cardinal 

 groove. 



Niagara limestone. Q-enoa, Ohio. 



'I .TF/ul'l 

 MONOMERELLA (ci'.f. ORBipUI<^KIS, .Billillgs). 

 Fig. 3. Protile view of an internal cast. 



Fig. 4. Another view of the same specimen; showing the internal features of the, pedicle-valve. The 

 cardinal gi-oove and the crescent are very consxiicuous, the umbonal chambers short, scarcely 

 reaching the hinge, and the later.al muscular impressions very shaj-ply developed. 

 Fig. .5. The opposite side of the same sjiecimen ; showing the low umbonal chamber, the strong crescent, 

 and incipient platform-vaults. '•    * ,: ,   



Niagara limestone. Bestmemi Qraftonoiif;^ Cedarimrg,. Wisconsin. 



'!■ '"'' 'i;!'] ' .■■. ,.r.v, , -.(1; .1 . ,' , , 



MONOMEItELtA PRISCA,'Brlliilo'S. 



Fig. (i. An internal cast of a portion of the pedicle-valve, in which the cardinal area is very high and the 



platform slightly unsymmetrical. 

 Fig. 7. The intei-ior of a pedicle-valve, drawn from a gutta-percha impression of the specimen represented 



in tig. 6, and slightly i-estored about the antei-ior portion. 

 Fig. 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 natu:-al cast. 

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



The crescent and lateral scars of the pkitfo-nn are unusually distinct. 

 Fig. 10. An internal cast of the pedicle-valve, in which the umbonal chambers are of unequal length. 

 Fig. 11. An internal cast of both valves, to which a portion of the mati-ix adheres ; showing the thickne.ss 



of the .shell in the umbonal regions. 



Fig. 12. An internal cast of a brachial valve, with indistinct radiating, prol)ably vascular furrows in the 

 anterior i-egion. 



Fig. 13. An internal cast of a brachial valve ; showing the characteristic markings of the shell. 



Niagara limestone. Hawthorne, Illinois. 



Fig. 



Fia 



MoNOMERELLA OrTONI. Sp. IIOV. 



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



defined. 



15. The counterpart of the same vahe, tVom a gutta-percha impression ; showing the broad deltidium 



without evidence of subdivi.sinn, the <-onspicuous cardinal slope and groove, the crescent and 

 platform scars and the pallial trunks with their ramifications. 

 Niagara limestone. Rising Sun, Ohio. 



