PLATE VIlA— Conlinaert- 

 Fig. 17. A caiilinal view of the same sperimeii. X S. 

 Fig. 18. An enlarg'ement of the surface of an internal cast. The granules may be casts of punctae on the 



inner layer of the shell. X !>• 

 Fig. 19. All enlarf>-enient of the external surface of the shell ; showing the sharp concentric striae crossing 



the alternating plications. X U- 

 Fig. 20. The central cardinal pcn-tion of the biacliial valve; showing the broaii, incurved delthyrium with 

 a thickeneil ai'ea at its base, and the absence of a cardinal process. X t>. 

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



Protokthis 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, 188.5, i)Iate v, figs. 20 a-c. 



Genus ORTHIDIUM, gen. nov. 



I'age '241. 



Ortiudium gemmicula, Billings. 



Figs. 22, 23, 24. Three view.s 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 Llvis ; in the upper part of the Limestone No. 2, Quebec group." (Billings.) 



Subgenus POLYTCECHIA, sub-gen. nov. 



Page 239. 



Clitambomtes (PoLYxaiCniA) APiCALis, AVIiitfield. 



Fig. 2G. The cardinal portion of the pedicle-valve. The deltidium has been I'emoved exposing the con- 

 cave spondylium (1) suppoi'ted by a median septum (s) and lateral septa (s'). The accessory 

 lateral ridges (s") are also seen. X 3. 



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

 partially enclosing the simple cardinal process. The crural jilates are very divergent and 

 nearly jiarallel 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. Fm't Cassin, Vermont. 

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



Genus SCENIDIU.M, Hall. 



(See Plate VII, Hg.-. 2il-3.1. 

 Pago 241. 



Ortiiis (Sce.mdiuji ??) Mekoi-e, Billiiio-.s. 

 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 biachial valve ; showing the simple cardinal process .and the crural plates. 

 The deltidial cavity is thickened transveraely, and from its center a low median ridge extends 

 forward. X 4. 



Hudson River group. Cincinnati. Ohio. 



SCENIDIU-M IIalli, Safluld. 

 Fig. 33. The exterioi- 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 pi-ocess in the bottom of the brachial delthyrium. X 4. 

 Fig. 35. The exterior of the brachial valve of the same specimen. X 4. 



Fig. 36. The interior of a brachial valve ; show-ing the triangular hinge-plate resting on the bottom of the 

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

 The (nitline of the valve is more elongate than in the pi-eceding specimen. X 4. 



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



