PLATE VIlA— Conlinued- 

 Fig-. 17. A cardinal view of the same specimen. X 3. 

 Fig-. 18. An enlargement of the surface of an internal cast. The granules may he casts of punctas on the 



inner layer of the shell. X 0. * 



Fig. 19. An enlargement of the external surface of the shell ; showing- the sharp concentric strise crossing 



the alternating plications. X B. 

 Fig. 20. The centi-al cai-diiial poi-tion of the brachial valve; showing the broad, incurved delthyrium with 

 a thickeneil ai-ea at its base, and the absence of a cardinal process. X 'J. 

 St. John gi-oup. /St. John, N. B. 



Pkotokthis Quacoensls, Matthew. 



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



St. John group. Portland, N. B. 

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



Genus ORTHIDIUM, gex. nov. 



Page-iil. 



Orthidium gemmicula, Billings. 



Figs. 22, 23, 24. Thi-ee views of this shell. (After Billings.) 



Fig. 25. The interioi- of- abi'achial valve ; showing the simple, erect cardinal jjrocess, coalesced with the 

 ci-ural plates. X 4. 



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



Subgenus POLYTQiCHIA, sub-gen. nov. 



Paso 239. 



Clitambonites (PoLTTQiCHiA) APiCALLS AVliittield. 



Fig. 26. The cai-Jinal portion of the jiedicle-valve. The deltidium has been removed exposing the con- 

 cave spondyliiun (1) supi_iorted by a median septum (s) and lateral septa (s'). The accessoi'y 

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



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

 partially enclosing the simjile cardinal process. The crui-al jilates are very divergent and 

 nearly parallel to the hinge-line. X 3. 



Figs. 28, 29, 30. Three views of the oi'iginal specimen of the si^ecies, the last showing the high cai'dinal 

 area and convex deltidium. X 3. 



Calciferous beds. Fort Cassin, Vtrmont. 

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



Genus SCENIDIUM, Hall. 



(See Plate VII, ligs. 29-3.'i. 

 Page 211. 



Oethis (Scenidium ??) Merope, Billings. 



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

 Fig. 32. The iiitei'ior of the brachial valve ; showing the simple cardinal process and the crural jilates. 



The deltidial cavity is thickened trans\-ersely, and fi-om its center a low median i-idge extends 



forwai'd. X 4. 



Hudson River group. Cincinnati, Ohio. 



Scenidium Halli, Saflbixl. 



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



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



with the slender process 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 ; showing the triangulaj- hinge-plate resting on the bottom of the 

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

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



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



