ORISKANY SANDSTONE. 4»r 



Fig. 7. The interior of a large ventral valve. 



Fig. 8. Enlargement of the plications, showing fine concentric striae. 



PLATE XCIX. 

 Fig. 1, 2 & 3. Casts of the dorsal valve of youflg and medium-sized specimens. 

 Fig. 4 & 5. Casts of larger individuals, showing the marks of the muscular impressions. 



In the smaller specimens the arching striae are rarely well shown in the cast 

 of the muscular impressions, while the longitudinal ones are well showUi 



Fig. 6 a. A specimen below the medium size, in which the marks of the muscular impression 



are well preserved, the transverse striae being as distinct as in older shells; 

 Fig. 6 b. Cardinal view of the same specimen. 

 Fig. 7 & 8. Casts of the ventral valve of very old specimens. 



Fig. 9 & 10. Casts of the dorsal valve, the latter figure presenting an unusual extension of 

 the beak. 



PLATE C. 



Fig. 1 a, b. Dorsal and ventral views of a very young specimen, showing the characters 

 already described. 



Fig. 2. A larger specimen. 



Fig. 3 a, b. Dorsal and ventral views of a larger specimen, showing the commencement of 

 the dichotomizing of the plications of the mesial fold and sinus. 



Fig. 4 a, h. Dorsal and ventral views of a specimen of medium size, in which the cardinal 

 angles are a little produced. 



Fig. 4 c. Profile view of the same specimen. 

 Fig. 4 d. Cardinal view of the same. 

 Fig. 4 «. Front view of the same. 

 Fig. 5. The interior of a dorsal valve. 



Fig. 6. The interior of a ventral valve, with an area of medium width) and a slight thickening 

 on each side of the muscular impression. 



Fig. 7. An old shell having the foramen partially closed by a callosity, and an excessive 

 thickening of the shell on the inside. The inner laminae of the shell are much 

 broken away. 



Fig. 8 a. The exterior of the ventral valve of a large individual, which preserves the exter- 

 nal markings in a good degree of perfection. 



Fig. 8 b. The interior of the same, showing the large area^ partially closed foramen, and 

 dental lamellae, while the inner lamina of the shell around the muscular im- 

 pression is exfoliated. 



There are certain variations of form noticed among the individuals in large collections of 



this fossil, which indicate a variety, or possibly a specific difiercnce. 



Geological position and locality. Everywhere in the Oriskany sandstone in New- 

 York; being one of the most constant, if not the most persistent fossil of the rock. 

 In the same rock in Maryland, Virginia, and elsewhere. 



