PLATE XLVIII-Contlnued. 



Vig. 19. An internal cast of the i)e<iicle-valve ; showing- sharply defined impressions of the adductor and 



didactor scars. 

 Vig. 20. An internal cast of the brachial valve ; showing the character of the adductor impression. 



Hamilton gri-oup. Shales of western New York. 

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



Cornifei-ous limestone. Columbus, Ohio. 

 Fig. 22. Dmbonal portion of the interior of a pedicle-valve ; showing the testaceous thickening over the 

 posterior portion of the muscular area. X 3. 



Hamilton group. Canandaigua Lake, N. Y. 

 Pig. 23. The hinge-plate viewed from in front, the ui)|ier surface of the plate being held in a horizontal 



position ; showing the concave surface of the plate an<l size of the crural lobes. X 3. 

 Pig. 28. The same sjiecimen viewed from above. X 3. 



Corniferous limestone. Coluinbus, Ohio. 

 Pig. 34. A portion of the brachidium, viewed from the doreal side ; showing the curvature of the spiral 

 volutions, the long straight crura, their attachment to the primary lamellse, and the foreshort- 

 ened loop. X 3. 

 Fig. 25. A portion of the external surface of the shell, enlarged to show the character of the spinules. 



Hamilton group. Western New York. 

 Pig. 26. The interior of a brachial valve ; showing some difference in the form of the hinge-plate from that 

 seen in figs. 23 and 28 ; also the adductor scare and vascular markings. 

 Hamilton group. Falls of the Ohio. 

 Fig. 27. The hinge-plate viewed from in fi-ont in the line of the bottom of the valve ; showing the elevation 



of its anteiior face and the sharp upward inclination of the crural bases. X 3. 

 Kg. 29. The articulating apparatus of conjoined valves ; showing the elevation of the hinge-plate and 



crural bases, and their relation to the teeth of the other valve. X 3. 

 Pig. 30. A profile of the brachial valve ; showing the recurvature of the hinge-plate and the projection of 



the crural bases. 

 Pig. 31. The interior of the same valve ; showing the upper face of the cardinal process and the muscular 



impressions. 

 Pig. 32. An enlargement of the cardinal portion of the i)edicle-valve ; showing the concave, completely 

 coalesced and solid deltidial plates, and the teeth. X 5. 

 Hamilton group. Western New York. 

 Pig. 33. A preparation of the brachidium i-etaining one-half of the spiral cones, and the ci°ui-a, primary 

 lAmellse and the loop complete. The lamellse of the brachidium are silicified and the lateral 

 branches of the loop bear along their inner or posterior mai-gins a series of fine, somewhat 

 irregular, branching processes, directed outward, toward or outside of the primary lamella;. 

 Fig. 34. An oblique view of the same specimen. X 3. (c.) 



lliis iieculiar structure has been observed in this single instance only. The shell in all external 

 and other features is indistinguishable from N. concinna, and the nature and value of this 

 peculiar variation in the structure of the loop is yet to be determined. (See page 146, fig. 132.) 



