PLATE XIa. 
(Figures 1, 2, 4-10,12-39 by G. B.Simpson; 3 by J. C. McConnell; 11 by E. Emmons.) 
Legend. D. Deltidium. 
t. Teeth. 
j. Cai'dinal process, 
fp. Crural plates, 
b. Dental sockets. 
mf. Muscular fulcra, 
s. Median septum, 
a. Adductor scars, 
r. Diductor scars, 
x. Cicatrix of attachment.!’ 
Genus STROPHOMENA, Rafinesque (de Blainville). 
(See Plates IX and IXa.) 
Page 245. 
Strophomena Wisconsinensis, Whitfield. 
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 pallial region. 
2. Profile of the same specimen ; showing the degree of retrorsion in the umbo of the pedicle-valve. 
Hudson River group. Wilmington, Illinois. 
For other illustrations, see Geology of Wisconsin, Yol. IV, plate xii, figs. 11-13. 
Strophomena filitexta, Hull. 
(See Plate IX, figs. 1-7; and Plate IXa, figs. 10-15.) 
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. 
4. Profile view ; showing the resupination of the lower valve and the great anterior deflection of the 
shell. 
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 group. Spring Valley, Minnesota. 
Strophomena nutans, Janies (Meek). 
(See Plate IXa, figs. 5-7.) 
6. The interior 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. 
Hudson River group. Wei.seburgh, Indiana. 
Strophomena sulcata, de Verneuil. 
(See Plate IX, figs. 8, 9.) 
8. The interior of a brachial valve; showing the peculiar character of the cai’dinal 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.) 
9. Cardinal view of an internal cast; showing the impression of the inner face of the cardinal process. 
Niagara group. Bridgeport, Illinois. 
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 short and erect, terminating 
abruptly. X 3. 
Niagara group. Waldron, Indiana. 
