PLATE LVI— Continned. 



Pllf. 31. An enlargeiueiit of Iho iinihonHl ixirtioii of the same Bpeciineii ; showing tlio deflection of the ver- 

 tioil aeptiform luinlin&l piocees, the charactci- of the hinge-plate, socket-wallci, ao<;ket8 and 

 crural apophyaeB. 

 Kigs. n, 18. 20, 21 are from the speciinenB upon which the characters of the genus were originally 

 eRtalilished. 

 , Hu(1w>n River grroup. Iron Ridge, Wisconsin. 



Fig. 22. The interior of an old brachial valve ; showing the deflection of the cardinal process and the 

 thickenetl hinge-jjlate. 



Rg. 28. The interior of a jiedide-valve in which the deltidial plates have been partially resorbed, leaving 

 the j>e«licle cavity open and exposing a well derined pedicle scar. The impression of the 

 diductor muscles is large and exi>and('d, indicatintr that the contraction of the scar with aee, as 

 seen in 6gure 17, is due to the encroachment of testaceous deposits upon the area of muscular 

 insertion. 



Hudson River group. Bichmimd, Indiana. 



Fin. 34-97. Dorsal, ventral, pi-ofile and frontal views of a young shell which, at an eai ly growth-stage, suf- 

 fered an interruption to the regular development of its jilications on both valves, the subsecjuent 

 shell-growth being irit^gular and showing but an imperfect development of the plications. The 

 cessation of uorinnl growth at the same stage on both valves would indicate that the production 

 of the al)norinal shell was due to a pathologic cotidition of the mantle, which has thtis repro- 

 duced in the later development of the valves, an elementary condition of growth. 

 Hudson River group. Near Cincinnati, Ohio. 



Genus CAMAROTCECPIIA, gen. nov. 



I'age 1J9. 

 CaMAROTCECIIIA FKINGILLA, Billillg.S. 



Figs. 38-30. Dorsal, profile and ventral views of a large and finely developed individual ; showing the ex- 

 ternal characters of the species. 



Anticosti group. Oull Cape, Anticosti. 



Genus RHYNCIIOTRETA, Hall. 



I'ago 1S5. 



Rhynciiotreta cuneata, Daliiuiii, var. Amekicana, Htill. 



Piga. 31-34. Dorsal, profile, ventral and fiontal views of an average example ; showing the contour of the 



shell, the character of the plication and fine concentric surface markings. 

 Plj. 86. Doreal view of the youngest individual observed ; .showing the open delthyrium, broad umbones 

 and the median sinus on the bi-achial valve. X 6. (After Brrchkr and Clarkb.) 

 Niagara group. Waldron, Indiana. 

 Figs. 36, 37. Ventral and dorsal views of an internal cast of conjoined valves ; showing, in figure 36, the 

 impression of the ]>edicle muscle, the diductor and adductor scars bounded by divergent ridges, 

 and, in figure 37, the cast of the deltidiiil cavity and the extent of the meiiian septum. 

 Niagara dolomites. Near Milwaukee, Wi.icon.nn. 



Fig. 38. The cardinal region of an adult specimen, enlarged ; showing the unusual size of the deltidial 

 plates, their outward flexion along the median suture, the apical and encroaching position of 

 the foramen. X 5. (After Bbechkr and Clarke.) 

 Niagara group. Waldron, Indiana. 



Rhynchotreta cune.\ta, Daliiiiin. 



Figs. 39, 40. Profile and dorsal views of a normal adult. (After Davidson.) 

 Weidock limestone. Dtidliy, England. 



Genus STENOSCHISMA, Conrad. 



Page 187. 

 StENOSCHISMA FORMOSA, Hall. 

 Flga. 41-43. Dorsal, profile and ventral views of the exterior. 

 Fig. 44. Frontal view of another example. 



Y\g. 45. Enlargement of the hinge-plate ; showing its deep median division, miiuite cardinal process, flat 

 cmral lobes and concave crura. X 4. 



Lower Heldei'bei-g group (See Vol. Ill, p. 286, pi, xxxv). Albany amnty, N. T. 



