PLATE LVI—Continued. 
Fig. 21. An enlargement of the umbonal portion of the same specimen; showing the deflection of the ver¬ 
tical septiform cardinal jirocess, the character of the hinge-plate, socket-walls, sockets and 
crural ajiophyses. 
Figs. 17,18.20, 21 are from the specimens upon which the characters of the genus were originally 
’ established. 
Hudson River group. Iron Ridge, Wisconsin. 
Fig. 22, The interior of an old bi-achial valve; showing the deflection of the cardinal process and the 
thickened hinge-plate. 
Fig. 23. The intei-ioi- of a pedicle-valve in which the deltidial plates have been partially resorbed, leaving 
the pedicle-cavity open and exposing a well defined pedicle scar. The impression of the 
diductor muscles is large and expanded, indicating that the conti'action of the scar with nse, as 
seen in figure 17, is due to the encroachment of testaceous deposits upon the area of muscular 
insertion. 
Hudson River group. Richmond, Indiana. 
Figs. 24-27. Dorsal, ventral, profile and frontal views of a young shell which, at an eai ly growth-stage, suf¬ 
fered an inteiTuption to the regular development of its plications on both valves, the subsequent 
shell-growth being irregular and showing but an imperfect development of the plications. The 
cessation of normal growth at the same stage on both valves would indicate that the production 
of the abnormal shell was due to a pathologic condition of the mantle, which has thus repro¬ 
duced in the latej- development of the valves, an elementary condition of growth. 
Hudson River group. Near Cincinnati, Ohio. 
Genus CAMAKOTCECHIA, gen. nov. 
Page 1S9. 
Camarot<echia fringilla, Billings. 
Figs. 28-30. Dorsal, xn’oflle and ventral views of a large and finely develojied individual; showing the ex¬ 
ternal characters of the species. 
Anticosti group. Gull Cape, Anticosti. 
Genus RHYNCIIOTRETA, Hall. 
Page ISa. 
Rhynchotreta cuneata, Daliiian, var. Americana, Hall. 
Figs. 31-34. Dorsal, xn'otile, ventral and frontal views of an average examjfle; showing the contour of the 
shell, the character of the plication and fine concentric surface markings. 
Fig. 35. Dorsal view of the youngest individual observed ; showing the open delthyrium, broad umbones 
and the median sinus on the brachial valve. X 6. (After Beecher and Clarke.) 
Niagara grouxi. Waldron, Indiana. 
Figs. 36, 37. Ventral and dorsal views of an internal cast of conjoined valves; showing, in figure 36, the 
iinxiression of the pedicle muscle, the diductor and adductor scars bounded by divergent ridges, 
and, in figure 37, the cast of the deltidial cavity and the extent of the median septum. 
Niagara dolomites. Near Milwaukee, Wisconsin. 
Fig. 38. The cardinal region of an adult si^ecimen, enlai-ged ; showing the unusual size of the deltidial 
plates, their outward flexion along the median suture, the ajiical and encroaching jiosition of 
the foramen. X 5. (After Beecher and Clarke.) 
Niagara grouxi. Waldron, Indiana. 
Rhynchotreta cuneata, Dulmiin. 
Figs. 39, 40. Profile and dorsal views of a noiinal adult. (After Davidson.) 
Wenlock limestone. Dudley, England. 
Genus STENOSCHISMA, Conrad. 
Page 187. 
StENOSCTTISMA FORMOSA, Hull. 
Figs. 41-43. Dorsal, jn’otile and ventral views of the exterior. 
Fig. 44. Frontal view of another examjAe. 
Fig. 45. Enlargement of the hinge-plate ; showing its deep median division, minute cardinal process, flat 
crural lobes and concave crura. X 4. 
Lower Helderberg grouxi (See Vol. Ill, ji. 236, pi. xxxv). Albany county, N. T. 
