PLATE XIa. 



(Figures 1, 2, 4-10, 12 3n by G. 15. Simpson; 3 by J. C. McConnei,!,; 11 by E. Emmons.) 



Ijegeiid. 



Fig. 

 Fig. 



Fig. 



I). 



t. 



J- 

 fp. 

 b. 



Deltidium. 

 Teeth. 



Cai'ilinal prcxn^ss. 

 Crural plates. 

 Dental sockets. 



inf. MiiKciilar fulcra, 



s. Median septum. 



a. Add uc^tor .scars. 



r. Diductor scai'S. 



X. Cicatrix of attac^hment.^' 



Genus STKOPHOMENA, Kafinesque (de Elainville). 



(See Plates IX and IXa.) 



Page 245. 



^TROl'HOMENA WiSCONSINENSIS, Whitfield. 



Fig. 1. Cardinal view of a specimen ; showing the broad cardinal area and deltidium of the pedicle-valve, 



the concavity of the brachial valve at its beak and its great convexity over the jiallial region. 

 Fig. 2. Pi'otile of the same specimen ; showing the degree of retrorsion in the umbo of the peiliclo-valvi'. 

 Hudson River group. Wilmington, Illinois. 

 For other illustrations, see Geology of Wisconsin, Vol. IV, plate xii, tigs. 11-18. 



Strophomena filitbxta, Hull. 

 (See Plate IX, tigs. 1-7 ; and Plate IXa, tigs. 10-1.5.) 

 Fig. 3. The interior of a brachial valve; showing the articulating apjiaratus anil the lobate va.scular (?) 

 ridges extending forward from the impression of the adductor muscles. 

 Hudson River group. Richmond, Indiana. 



SXROPHOMENA FLUCTUOSA, BilHugS. 



Fig. 4. Profile 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 pre.served. X 3. 



Hudson River group. tSpriiig Valley, Minne.'iota. 



Strophomena NUTANS, Jiiiues (Meek). 

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



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



7. Profile of the same specimen ; showing the great elevation of the interrupted ridges surrounding 

 the muscular area, and the median thickening on the anterior margin. 



Huilson River group. Weiseburgh, Indiana. 



Strophomena SULCATA, de Veiiieiiil. 



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



8. The interioi' of a lii'achial valve ; showing the peculiar character of the cardinal process, the 

 , strong curved crural plates, and the thickened muscular area. 



Hudson River gi-oup. Oxford, Ohio. 



Genus ORTHOTHETES, Flscher de Waldheim. 



(See Plates IX and IXa.) 

 Page 253. 



Orthothetes subplana, Coiinul. 



(See Plate IX, figs. 21-'J4.) 

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



Niagara 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 developeil deltidium, and the cmral plates are short and erect, terminating 

 ahruptly. X 3.' 



Niagara group. Waldron, liidiaiia. 



