PLATE IV c— Continued. 

 MONOMERELLA EgANI, Sp. IIOV. 



Fig. 16. A bi-achial valve ; showing the strongly developed cai-dinal area, the narrow crescent and the 

 platform scars. The umhonal cavity is filled hy a thick deposit of testaceous matter. 

 Niagara limestone. Grafton, Wincon.siii. 



MONOMERELLA OVATA, Viir. LATA, WlliteUVCS. 



Fig. 17. The bi-achial side of an internal cast, believed to belong to this variety. The umbonal cavity is 

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

 ing furrows in front of the platform are probably to be regarded as vasculai- markings. 



Fig. 18. The opposite side of the same specimen ; showing the very .short umbonal chambers. 

 Niagara limestone. Haiotlioi-ne, Illinuis. 



