PLATE XX. 
ORTHIS, Dalman. 
|Page 227. 
Orthis flabellites, Hull. 
(See Plate V, figs. 37-41.) 
Fig. 1. An internal cast of the pedicle-valve ; showing very distinctly the character of the muscular scar, 
and the simple plication of the surface. 
Niagara group. Hamilton, Ontario .! 
Orthis punctostriata, Hull. 
Page 217. 
Figs. 2, 3. Two views of a slightly distorted individual, retaining both valves. 
Fig. 4. An enlargement of the surface ; showing the rows of superficial punctse between the striae. 
Niagara group. Lockport, N. Y. 
Orthis (Rhipidomella) Burlingtonensis, Hall. 
(See Plate VIa, fig. 13.) 
Fig. 5. An exfoliated pedicle-valve ; showing very distinct impressions of the adductor and diductor scar. 
Fig. 6. Posterior view of the cardinal process and crural plates of the brachial valve. X 4. 
Chert of the Burlington limestone. Pike county, Missouri. 
Orthis (Rhipidomella) Michelini, LeveilR. 
Fig. 7. The interior of the pedicle-valve ; after Davidson. (British Carboniferous Brachiopoda, pi. xxx, 
fig. 11.) 
Carboniferous limestone. Ayrshire. 
ORTHOTHETES, Fischer de Waldheim. 
Page 253. 
Orthothetes deformis, Hall, var. sinuata, var. nov. 
(See Plate IX, fig. 32.) 
Figs. 8, 9. Two views of a specimen ; showing the distortion of the beak and the median sinus on the bra- 
valve, which is a characteristic feature of some species of Derbya. Further examination will 
probably prove this form to be a distinct species. 
Lower Helderberg group. Cumberland, Md. 
DERBYA, Waagen. 
Page 261. 
Derbya Correana, Derby. 
(See Plate XI, figs. 18-22.) 
Figs. 10, 11. Two views of the umbonal portion of an internal cast of a pedicle-valve, broken to show the filling 
of the umbonal cavity between the united dental plates. Fig. 10 also shows the impression of 
the median septum supporting these plates. X 2. 
Carboniferous limestone. Itaituba, Brazil. 
Derbya crassa, Meek and Hayden. 
(See Plates X, XIb, XIc.) 
Figs. 12, 13. The interior of both valves of the same specimen ; showing the muscular scars, median septum 
and cardinal process. 
Upper Coal Measures. Near Kansas City, Missouri. 
