ORISKANY SANDSTONE. 431 



Merista lata ( n, s.). 



Plate CI. Fio. 3 a - m. 



Shell subovate or subquadrate, sometimes longitudinally oval. Ventral 

 valve longitudinally arcuate, gibbous in the middle, the greatest con- 

 vexity about halfway from beak to base, with a shallow depression in 

 front which extends upwards not more than one-third the length of the 

 valve, and is not seen in young specimens : beak elevated and closely 

 incurved over the opposite one. Dorsal valve abruptly elevated in the 

 middle, and somewhat flattened at the sides. 



Surface marked by a few strong lamellae indicating periods of growth, 

 and by finer parallel striae ; while the entire surface is covered by faint 

 radiating striae. 

 The casts of this species, in the dorsal valve, show the mark of a 



median septum reaching more than halfway to the base. The muscular 



impressions in the cast of the ventral valve are large and strongly defined. 



This species is intermediate in form between M. arcxtata and M. suhquadrata of 

 the Lower Helderberg group; being less sinuate and more distinctly striate than 

 the former, and less produced in front, with a greater elevation on the dorsal valve, 

 than the latter. 



The radiating striae are more distinctly shown in the figures than they should be, 

 and appear much as the specimen does under an ordinary lens. 



Fig. 3 a. Ventral valve of a young specimen. 



Fig. 3 b. Dorsal view of a large specimen. 



Fig. 3 c, d, e. Profile, ventral and front views of the same specimen. 



Fig. 3y. A ventral view of an intermediate form. 



Fig. 3 g, h. Dorsal and ventral views of a cast of this species. ( The mark of the median 

 septum is not shown in some of the figures.) 



Fig. 3 J. Profile view of the same. 



Fig. 3 k. Ventral view of an imperfect cast, showing a larger muscular area. 



Fig. 3 /. Ventral view of a large individual of this species. 



Fig. 3 m. Profile view of the same. 



Geological position and localities. In the Oriskany sandstone : Albarty and Scho- 

 harie counties ( New- York), and Cumberland ( Maryland ). 



