PLATE VII A— Continueil- 

 Fig. 17. A caiilinal viow of the same specimen. X 3. 

 Fig. 18. All enlarfjoment, of the surface of an internal cast. The granules may be casts of i>inK-tae on the 



inner layer of the shell. X 0. 

 Fiu-. 19. An enlargement of the external surface of the shell ; showing the sharp concentric striie crossing 



the alternating plications. X 6. 

 Fig. 20. The central cardinal portion of the brachial valve ; showing the broad, incurved delthyrium Vi^ith 

 a thickene<l area at its liase, and the absence of a cardinal process. X ti. 

 St. John group. St. John, N. B. 



Pkotorthis Quacoensis, Matthew. 



Fig. 21. The interior of a pedicle-valve. X 2. 



St. John group. Portland. N. B. 

 For other illustration, see Trans. Royal Society of Canada, Sec. IV, 1885, plate v, figs. 20 a-c. 



Genus ORTHIDIUM, gen. nov. 



Page 241. 



Orthidium gemmicula, Billings. 

 Figs. 22, 23, 24. Three views of this shell. (After Billings.) 



Fig. 25. The interior of a brachial valve ; showing the simple, erect cardinal process, coalesced with the 

 crural plates. X 4. 



" Point LM.s ; in the upper part of the Limestone No. 2, Quebec group." (Billings.) 



Sub-genus POLYTGECHIA, sub-gen. nov. 



Page 239. 



Clitambonites (Polytcechia) apicalis, Whittiekl. 



Fig. 26. The cardinal portion of the pedicle-valve. The deltidium has been i-emoved exposing the con- 

 cave spondylium (1) supported l)y a median septum (s) and lateral sejita (s'). The accessory 

 lateral ridges (s") are al.so seen. X 3. 



Fig. 27. The cardinal portion of the lirachial valve ; showing in profile the lateral portions of the deltidium 

 partially enclosing the simjile cardinal pi-ocess. The crui'al plates are very divergent and 

 nearly parallel to the hinge-line. X 3. .^ 



Figs. 28, 29, 30. Three views of the original specimen of the .species, the last showing the high cardinal 

 area and convex deltidium. X 3. 



Calciferous beds. Fort Cassin, Vermont. 

 For other illustration, see Bull. American Mus. iN'at. Hist., Vol. I, plate xxiv, figs. 1-5. 



Genus SCENIDIUM, Hall. 



(See Plate VII. tigs. 29-35. 

 Page 241. 



Ortiiis (ScENiDiuM ??) Merope, BiUillgS. 

 Fig. 31. A view from the brachial valve ; showing the area and foramen of the opposite valve. X 4. 

 Fig. 32. The interior of the bra<;hial \-alve ; showing the simple cardinal process and the crural plates. 

 The (ieltidial cavity is thickened transversely, .and ti-om its center a low median ridge extends 

 forward. X 4. 



Hudson River group. Cincinnati. Ohio. 



SCENIDIU.M IIaI.LI, Sllffdld. 

 Fig. 33. The exterior of the pedicle-valve ; showing the elevation of the beak. X 4. 

 Fig. 34. A cardinal view of the same specimen ; showing the area and open delthyrium on both valves, 



with the slender process in the bottom of the brachial delthyi-ium. X 4. 

 Fig. 35. The exterior of the i)rachial valve of the same specimen. X 4. 



Fig. 36. The interior of a l)rachial valve ; showing the triangular hinge-plate I'esting on the bottom of the 

 valve, and thi; low median septum extending beyond the middle of the shell. 

 The outline of the valve is more elongate than in the preceding sijecimen. X 4. 



Trenton limestone. (" Glade Limestone " of Safford.) Nashville, Tennessee. 



