

PLATE X. 







(Figures 1-14, 16, 18-23 by R. P. Whitfikld; 



16, ITbyF. B. Mkek.) 



u. 



A. 

 I. 

 t. 

 d. 

 b. 



Deltidium. 



Cardinal area. 



Umbo-lateral slopes. 



Teeth. 



Dental lamellae. 



Dental sockets. 



j . Cardinal process, 

 c. Crura. 



s. Median septum, 

 a. Adductor scars. 

 r. Diductor scare. 



Leg'end. 



Genus ORTHOTHETES, Fischer de Waldheim. 



I'age ^3. 



Orthothetes Chemungensis, Conrad, var. Pandora, Billings. 



Fig. 1. A view from the brachial valve ; showing the cardinal area and deltidium of the opposite valve. 



Corniferous limestone. Caledonia, N. Y. 

 Fig. 2. The interior of a brachial valve, di'awn from a gutta-percha impression ; showing the muscular 

 area, and, imperfectly, the cardinal process. 

 Schoharie grit. /Schoharie, N. Y. 

 Fig. 3. The interior of a larger brachial valve ; showing with more accuracy the crural plates and the 

 adductor scars. 

 Plate IX, Fig. 30. — An enlargement of a part of the hinge area, the cardinal process, crural 

 plates and muscular impressions of the preceding specimen. 

 Fig. 4. The interior of a pedicle-valve ; showing the low deltidium, with unsupported teeth, the adductor 

 and diductor scars. 



Corniferous limestone. Caledonia, N. Y. 

 Fig. 5. The interior of a pedicle-valve, drawn from a gutta-percha impression. 



Schoharie grit. Schoharie, N. Y. 

 Fig. 6. An internal cast of the pedicle-valve ; showing the flaljellate diductors and the impression of a 

 broad median i-idge. 



Corniferous limestone. Western New York. 



Orthothetes Chemungensis, Conrutl, var. alternata, Hall. 



Fig. 7. The interior of the brachial valve ; showing the characteristic cardinal pi-ocess. 

 Hamilton gioup. Canandaigua Lake, N. Y. 



Orthothetes Chemungensis, Conrad, var. arotostriata, Hall. 



Fig. 8. An internal cast of the pedicle-valve ; showing the impression left by the rather unusual develop- 

 ment of the ridges surrounding the adductor scars and uniting in the median line separating 

 the diductors. This ridge is not a septum but merely the thickening of the edges of the mus- 

 cular scars. X 2. 



Hamilton group. Western Neio York. 



Orthothetes Chemungensis, Conrad. 



Fig. 9. A cardinal view of a specimen ; showing the iri-egular form of the shell, the usual width of the 

 area, and the character of the deltidium. 



Chemung group. South-western New York. 

 For further illustration, see Palaeontology of New York, Volume IV, plates ix and x. 



Genus DERBYA. Waagen. 



Page 261. 



Derbya orassa, Meek and Hayden. 



Fig. 10. The interior of a brachial valve ; showing the cardinal process and muscular impressions. The 

 former is represented as having the lobes upon its edge and too acutely angular. 



Fig. 11. The interior of a pedicle-valve, with strong median septum dividing the muscular area, and 

 highly developed teeth projecting into the cavity of the valve. 

 Coal Measures. Near Winterset, Iowa. 



