PLATE IV c— Continued. 

 MONOMERELLA EgANI, Sp. IIOV. 



Fig. 16. A brachial valve ; showing the strongly developed cardinal area, the narrow crescent and the 

 platform scars. The nmhonal cavity is filled by a thick deposit of testaceous matter. 

 Niagara limestone. Chrafton, Wucmisin. 



Monomerella ovata, V!ir. lata, Whiteave.s. 



Fig. 17. The brachial side of an internal cast, believed to belong to this variety. The umbonal caWty is 

 broad but well defined, the crescent and platform scars strongly developed. The three diverg- 

 ing furrows in front of the platform are pi'obably to be regarded as vascular markings. 



Fig. 18 The opposite side of the same specimen ; showing the vei'y short umbonal chambei-s. 

 Niagara limestone. Hawthonif, Illinois. 



