(Figures 



Lefjend : 



PLATE XXVIII. 



48, 4'.i copies; 2-5, 8-i;i, l.i, il--J9, 31-33, 43, .'iO, 5-,: by R. P. Whitfield; 6, 7, 14, 16-20, 30, 34-42, 44-47, 51, 



53, 64 by E. Emmons. 



D. Deltidiuiii. 

 F. Foramen. 

 d. Dental lamellae, 

 s. Median septum, 

 s'. Callosity in dellhyrinm. 



T. Tubular edge of the inedinn .seiitum. 

 j. (Cardinal proces.a. 



b. Dental sockets. 



c. Soi;ket-walls. 



Genus CYRTIA, Dalman. 



Page 40. 



Cyrtia expokrecta, \\'alileiil)fig. 



Fig. 1. The exterior of a normal example. (After Davidson.)^ 



Figs. 48, 49. {On bottom line of jilate.) Enlarged views; showing the deltidiai covering, foramen and elon- 

 gate foraminal groove. In fig. 49 the foramen has been normally or casually closed. (After 

 D.WIDSON.) 



Fig. 51. Transverse section of the umbonal region; showing the dental lamellae resting on the bottom of 

 the valve and the thickening at their union with the deltidiai covering. 

 Wenlock limestone. England. 



Cyrtia exporrecta, Wnlilcnlicig, v.ir. arrecta, Hall and Whitfield-. 



Figs. 2, 3. Cardinal and profile views of the original specimen. 

 Niagara group. Louisville, Kentucky. 



Cyrtia radians, .sj). nov. 

 See Plate 21. 

 Figs. 4, 5. Cardinal and front views of a small example ; showing the finely radiate-lineate exterior. 

 Figs. 50, 52. (On bottom line of plate.) The central cardinal area of two examples; showing the direct, 

 circular foramen and the elongate foraminal groove. X 3. 

 Clinton group. Rochester, N. Y. 



Genus CYRTINA, Davidson. 



Page 43. 

 CyR'IIXA ROSTR.4TA, Hall. 



See Plate 25. 



Fig. 6. The interior of a pedicle-valve; .showing the junctionof the dental lamellit with the meilian sejitum. 

 Oriskany sandstone. Cuinberland, Maryland. 



Cyktina niPLiCATA, Hall. 



Fig. 7. A view of the pedicle-valve of a silicified internal cast ; showing a portion of one of the spirals. 



Corniferous limestone. Drift at Ann Harbor, Michigan. 

 Fig. 8. A cardinal view of conjoined valves ; showing the imperforate deltidium. 

 Figs. 9, 10. Front and dorsal views of the same specimen. 



Schoharie grit. Schoharie, jY. Y. 



Cyktina cuiivilineata, White. 



Fig. 11. A view of a large specimen ; showing the plicated median fold of the Ijiachial valve, and the dis- 

 torted umbo of the opposite valve. 

 Fig. 12. A profile of the same specimen. 



