PLATE LVl— Continued. 



Fig-. '21. An enlargement of the umbonal portion of the same specimen; t^howing the deflection of the ver- 

 tical septiform cardinal process, the character of the hinge-plate, socket-walls, sockets and 

 crui-al apophyses. 

 Figs. 17, 18. 20, 21 ai-e from the specimens upon which the characters of the genus were originally 

 established. 

 , Hudson River group. Irun Ridge, Winconsin. 



Fig. 22. The interior of an old brachial valve ; showing the deflection of the cardinal pro(-ess and the 

 thickened hinge-plate. 



Fig. 23. The interior of a pedicle-valve in which the deltidial plates have been i)artially 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 contraction of the scar with see, as 

 seen in figu?e 17, is due to the encroachment of testaceous deposits upon the area of muscular 

 insertion. 



Huds(m River gi'oup. Richmimd, Indiana. 



Figs. 24-27. Dor.sal, ventral, protile and frontal views of a young shell which, at an early growth-stage, suf- 

 fered an iiiteri'uption to the regular development of its plications on both valves, the subsequent 

 shell-growth lieing irregular and showing Imt an impeifect development of the plications The 

 cessation of noi'mal gi'owth at the same stage on both valves would indicate that the production 

 of the abnormal shell was due to a pathologic condition of (he mantle, which has thus repro- 

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

 Hudson River group. Near Cmcinnati, Ohio. 



Genus C.^MAROTOSCHIA, gen. nov. 



I'age ISO. 

 CaMAROT(ECI1IA FRINGILLA, Billing.s. 



Pigs. 28-30. Dorsal, protile and ventral views of a large and finely developed imlividual ; showing the ex- 

 ternal characters of the species. 



Anticosti group. Quit Cape, Anticosti. 



Genus EHYNCIIOTRETA, Hall. 



I'aKo 1S5. 

 IkllYNCIIOTRETA CUNEATA, Dilllliail, Vllf. AMERICANA, Htlll. 



Figs. 31-34. Dor.sal, protile, ventral and frontal views of an average example; showing the contour of the 



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

 Fig. S."). Dorsal view of the youngest individual observed ; showing the open delthyrium, broad uml>ones 

 .and the median sinus on the brachial valve. X t). (After Beecher and Clarke.) 

 Niagara group. Waldron, Indiana. 

 Figs. 36, 37. Ventral and dorsal views of an internal cast of conjoined valves ; showing, in figure 3G, the 

 impression of the pedicle muscle, the diductor and adductor scars Ijouniled by divergent ridges, 

 and, in figure 37, the cast of the deltidial cavity and the extent of the medijin .septum. 

 Niagara dolomites. Near Milwaukee, Wiscon.mi. 

 Fig. 38. The cardiual region of an adult specimen, enlarged ; showing the uuusual size of the deltiilial 

 plates, their outward flexion along the median suture, the apical and enci'oaching position of 

 the foramen. X 5. (After Beecher and Clabkij:.) 

 Niagara group. TValdro7i, Indiana. 



Rhynchotreta cuneata, Daliutiii. 



Figs. 39, 40. Profile and dorsal views of a noimal adult. (After David.son.) 

 Wenlock limestone. Dudley, England. 



Genus STENOSCHISMA, Conrad. 



Page 1S7. 

 StENOSCHISMA FORMOSA, Hall. 

 Figs. 41-43. Dorsal, profile and ventral views of the exlei-ior. 

 Fig. 44. Frontal view of another examjile. 



Fig. 45. Enlargement of the hinge-plate ; showing its deep median division, minute cardinal pi'ocess, flat 

 crural lobes and concave (uura. X 4. 



Lower Helderlierg group (See Vol. Ill, p. 23G, pi. xxxv). Alliany county. N. T. 



