BRACHfOPODA. 41 



entire surface, but rarely resolvable into separate scars. Anterior septum 

 indistinct, usually imperceptible. Pallial sinu.ses sometimes clearly defined. 



Brachial valve with tlie umbo seldom conspicuously developed, the umbonal 

 cavity being usually simple and deep, but sometimes filled by a deposition of 

 shelly matter. Crescent sharply impressed over the narrow canlinal slope, 

 and produced into broad, indistinct, composite terminal scars. Platform usually 

 well developed iuid broadly excavate on its anterior walls, but sometimes 

 scarcely discernible. Anterior and lateral scars sharply defined, giving a tri- 

 partite appearance to the platform. Anterior septum more strongly developed 

 than in the opposite valve. 



Type, Monomerella prisca, Billings. 



Observations. The very rich material that has been before us in the study 

 of this genus, has added a number of interesting and previously unknown forms, 

 and the variations from the type-species shown by these, has made it neces- 

 sary to modify in some respects the diagnosis of MoNOMEUt;LL.\, as given by 

 Davidson and King. At the date of their work but four species were known, 

 and the four additional species, which, for convenience in discussing the genus, 

 are described in the supplement to this volume, present some important 

 deviations from the type-form, and necessarily broaden the scope of the genus. 

 Mr. Billings' species, Monomerella prisca and M. orbicnlaris, from the Guelph 

 limestone at Hespeler, Canada, were first illustrated by the English writers, 

 who also added two new species, M. Walmskdli, from the Aymestry limestone 

 of Gotland, and M. Lindstrmmi, from the Wenlock near Wisby The former of 

 these Gotland species is very closely allied to M. prisca. Indeed, we are com- 

 pelled to include under this latter name, American forms which vary so con- 

 siderably in the elevation of the cardinal area, size of the umbonal chambers 

 of the pedicle-valve, and the depth of the umbonal cavity in the brachial valve, 

 that with this broader conception of the specific characters of M. prisca, it 

 would be easy to consider M. Walmstedti as conspecific. The specimens of M. 

 prisca, from the dolomites of the Niagara group, at the Rising Sun quarries, 

 Wood county, Ohio ; Hawthorne, Port Byron and Cicero, Illinois ; and near 



