PLATE XLVIII—Continued. 
Fig. 19. An internal cast of the pedicle-valve; showing sharply defined impressions of the adductor and 
diductor scars. 
Fig. 20. An internal cast of the brachial valve ; showing the chai-acter of the adductor impression. 
Hamilton group. Shales of western New York. 
Fig. 21. The interior of a pedicle-valve. X 2. 
Corniferous limestone. Columbus, Ohio. 
Fig. 22. Umbonal 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. T. 
Fig. 23. The hinge-plate viewed from in front, the ujjper surface of the plate being held in a horizontal 
position ; showing the concave surface of the plate and size of the crural lobes. X 3. 
Fig. 28. The same specimen viewed from above. X 3. 
Corniferous limestone. Columbus, Ohio. 
Fig. 24. A portion of the brachidium, viewed from the dorsal side; showing the curvature of the spiral 
volutions, the long sti'aight crura, their attachment to the primary lamellae, 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. Westei'n New York. 
Fig. 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 scars and vascular markings. 
Hamilton group. Falls of the Ohio. 
Fig. 27. The hinge-plate viewed from in front in the line of the bottom of the valve ; showing the elevation 
of its anterior face and the sharp upw’ard inclination of the crural bases. X 3. 
Fig. 29. The articulating ajii^aratus 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. 
Fig. 30. A profile of the brachial valve ; showing the recurvature of the hinge-plate and the projection of 
the crural bases. 
Fig. 31. The interior of the same valve ; showing the upper face of the cardinal process and th^ muscular 
impressions. 
Fig. 32. An enlargement of the cardinal portion of the pedicle-valve ; showing the concave, completely 
coalesced and solid deltidial plates, and the teeth. X 5. 
Hamilton gi'oup. Western New York. 
Fig. 33. A preparation of the brachidium retaining one-half of the spiral cones, and the crura, primary 
lamellae and the loop complete. The lamellae of the brachidium are silicified and the lateral 
branches of the loop bear along their inner or posterior margins a series of fine, somewhat 
irregular, branching processes, directed outward, toward or outside of the primary lamellae. 
Fig. 34. An oblique view of the same specimen. X 3. (c.) 
This peculiar structure has been observed in this single instance only. The shell in all external 
and othei- 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.) 
