PLATE XIa. 



(Kigures i, 2, 4-10, 12 at by G B Simfson; 3 by J. C. McConnbll; 11 by E. Emmons.) 



Legend. D. Delti.Uinn. mf. MusiMilar fult-ra. 



t. Teeth. s- Median seiiUiiii. 



j. Cariliiial |ir(ice.-<s. a. AdciiK'tor seai-.s. 



fp. Crural plates. r. Didiictor scars, 



b. Dental socket.s. x. Cicatrix of attachment.^' 



Genus STUOrHOMENA, Kai<'inesque (dk Hi.ainvilm:). 



(See Plates IX and IX A. J 

 Page 245, 



Stkoi'homkna \yiscoNsiNENSis, Whitfiold. 



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



the concavity of the bi'achial valve at its beak and its great convexity over the pallial I'egion. 

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

 Hudson River group. Wilmington, Illinois. 

 For other illustrations, see Geology of Wisconsin, Vol. IV, plate xii, tigs. U-13. 



StKOPHOMENA FILITEXTA, Hall. 



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

 Fig. 3. The interior of a brachial valve; showing the articulating apparatus and the lol>ate vascular (f) 

 ridges extending forward from the impi-ession of the adductor muscles. 

 Hudson River group. Richmond, Indiana. 



Stkophomena fluctuosa, Billiiigtf. 



Fig. 4. Protile view ; showing the resupination of the lower valve and the great anterior deflection of the 



shell. 

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



gi-owth 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 group. filp7-inff Valley. Minnesota. 



Stkophomena nutans, Junies (Mei'U). 



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

 Fig. H. The interior of a pedicle-valve. 



Fig. 7. Profile of the same specimen ; showing the great elevation of the interrui)teil ridges surrounding 

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

 Hudson River group. Weiseburgh, Indiana. 



Stkophomena sulcata, cle VfriK-iiii. 



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

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

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

 Hudson River group. Oxford, OMo. 



Genus ORTHOTHETES, Fischer de Waldheim. 



(See Plates IX and IXa.) 



Page 253. 



Oktuothetes SUBPLANA, CoiHiwl. 



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



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



Niagara group. Bridgeport. Illitiois. 

 Fig. 10. The cai-ilinal area of a brachial valve, enlarged. The lobes of the cardinal proc^ess are enclosed 

 behin.l by a well developed cieltidiuni, and the crural plates are short and erect, terminating 

 abruptly. X 3.' 



Niagara grou] I. f\'aldro7i, Indiana. 



