446 Systematic Paleoxtology 



" Surface marked by a few strong lamellje 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." Hall, 1859. 



This species shows considerable variation in outline, in the development 

 of the sinus, and the prominence of the dorsal fold. Some specimens are 

 less transverse than is typical, the sinus may be nearly obsolete and the 

 dorsal fold in the anterior region considerably reduced. Such specimens 

 approach M. symmetrica, but none have been seen that are so symmetrical 

 as that species. As a rule, however, M. lata is readily distinguished from 

 the associated transverse forms by its large size, broad shallow ventral 

 sinus, and the ridged dorsal shell. 



This widely distributed and common Oriskany Meristella is found 

 usually as internal casts. 



Length 3 cm. ; width 2.8 cm. 



Occurrence. — Helderberg Formation, Becraet Member. Cherry 

 Eun, West Virginia. Oriskany Formation, Eidgely Member. Cum- 

 berland, Maryland ; Warren Point, Pennsylvania. 



Collections. — Maryland Geological Survey, American Museum of 

 N'atural Histor}'. 



Meristella lentiformis Clarke 

 Plate LXXIV, Figs. 12, 13 



Meristella lentiformis Clarke, 1900, Mem. N. Y. State Mus., vol. iii. No. 3, 

 p. 44, pi. vi, figs. 5-11. 



Description. — "Shell unequally convex; outline transversely oval; 

 pedicle valve with short incur\ cd beak and narrow cardinal slopes bordered 

 by obtuse cardinal ridges diverging from the beak. Umbo slightly convex 

 or flat, the surface sloping to the sides very gradually. A median sinus 

 starts at the umbo and rapidly broadens, producing a general depression in 

 the pallial region, which is rather sharply deflected. The sinus is pro- 



