PLATE XIa. 



(-figures 1, -2, i-lO, 12-39 by G. B._Simpson; 3 bj' J. C. McConnell; 11 by E. Emmuns.) 



Legend. D. Deltidiuni. mf. Muscular fulcra, 



t. Teeth. s. Median septum. 



j. Cardinal process. a. Adductor scars, 



fp. Crural plates. r. Diductor scars, 



b. Dental sockets. x. Cicatrix of attachment. 3' 



Genus STKOPHOMENA, Eafinesque (de Blainville). 



(See Plates IX and IXa.) 

 Page 246. 



Strophomena Wisconsinensis, Whitfield. 



Pig. 1. Cardinal view of a specimen ; showing- the broad cardinal ai'ea and deltidium of the pedicle-valve, 



the concavity of the bi-achial valve at its beak and its gi-eat convexity over the pallial region. 

 Fig. 2. Profile of the same specimen ; showing the degree of retrorsion in the umbo of the pedicle-valve. 

 Hudson River gi'oup. Wilmington, Illinois. 

 Foi' other illustrations, see Geology of Wisconsin, Vol. IV, plate xii, figs. 11-13. 



Strophomena filitexta, H;ill. 



(See Plate IX, figs. 1-7 ; and Plate IXa, figs. 10-15.) 

 Fig. 3. The interior of a brachial valve ; showing the articulating apparatus and the lobate vascular (?) 

 ridges extending forward from the impression of the adductor muscles. 

 Hudson River group. Richmond, Indiana. 



Strophomena fluctuosa, Billings. 



Fig. 4. Pi'ofile view ; showing the resupination of the lower valve and the great anterior deflection of the 



shell. 

 Fig. 5. The cardinal area of the pedicle-valve, enlarged ; showing the deltidium, the striated path of 



growth of the dental ridges, and at (t) the pectinated margin of one of the teeth, the other not 



being well preserved. X 3. 



Hudson River gioup. Spri^ig. Valley, Minnesota, 



Strophomena nutans, Janies (Meek). 

 (See Plate IXa, figs. 5-7.) 

 Fig. 6. The intei-ior of a pedicle-valve. 



Fig. 7. Pj'ofile of the same specimen ; showing the great elevation of the intei-rupted i-idges surrocmding 

 the musculai' ai'ea, and the median thickening on the anterioi' mai'giu. 

 Hudson River group. Weiseburgh, Indiana. 



Strophomena sulcata, de Vemeuil. 

 (See Plate IX, figs. 8, 9.) 

 Fig. 8, The interior of a brachial valve ; showing the peculiar chai'acter of the cardinal process, the 

 strong curved crural plates, and the thickened muscular area. 

 Hudson River group. Oxford, Ohio. 



Genus ORTHOTHETES, Fischer de Waldheim. 



(See Plates IX and IXa.) 

 Page 253. 



Orthothetes subplana, Conrad. 



(See Plate IX, figs. 21-24.) 

 Fig. 9. Cardinal view of an internal cast; showing the impression of the inner face of the cardinal process. 



INiagara group. Bridgeport, Illinois. 

 Fig. 10. The cardinal area of a brachial valve, enlarged. The lobes of the cardinal process are enclosed 

 behind by a well developed deltidium, and the crural plates are shoi't and erect, terminating 

 abruptly. X 3. 



Niagara grouji. Waldron, Indiana. 



