448 Systematic Paleontology 



extension. Dorsal valve decidedly rectangular posteriorly, regularly con- 

 vex excepting in the anterior third, where it is modified by a depressed 

 convex and narrow medial fold. Surface usually marked by strong varices 

 of growth. Interior characters very much like those of M. lata. 



As a rule, species of Meristella are very variable in form, and this is the 

 case with M. rostellata. However, the elongate form will separate it from 

 M. lata, and the presence of a fold and sinus distinguish it from 21. 

 symmetrica. M. walcotti is a more obese species and less wide in the 

 anterior region than M. rostellata. M. ( ?) vascularia agrees in shape very 

 well with this species, but differs in not having the great incised muscular 

 pit of most Meristellas. M. princeps of the New York Becraft zone seems 

 to be the nearest in form, but the sinus and fold are far more pronounced. 



Length 5 cm. ; width 4.5 cm. 



Difficulties are always experienced in identifying small or immature 

 specimens of large Meristellas, and this is particularly the case with M. 

 rostellata. Some specimens will undoubtedly be mistaken for small elon- 

 gate individuals of M. lata, and because of the imperfect preservation one 

 cannot be certain of their reference to either species. With mature ex- 

 amples there need be no great difficulty. 



Occurrence. — Oriskany Formation, Eidgely Member. Cumberland. 



Collections. — Maryland Geological Survey, U. S. National Museum. 



Meristella symmetrica n. sp. 

 Plate LXXIV, Figs. 16-19 



Description. — The large size, thin shell, nearly round outline, sym- 

 metrical and regularly biconvex form, without fold or sinus, distinguish 

 this species from the associated Meristellas. Some specimens are more 

 elongate than others, but none can be said to be elongate. The average 

 size is 38 mm. long by 35 mm. wide, but a specimen found by Mr. Gordon 

 measures 60 mm. in length by 60 mm. in width. The ventral muscular 

 pit is somewhat narrower and less deeply excavated than in M. lata. 



Occurrence. — Oriskany Formation, Eidgely Member. Cumberland. 



Collections. — Maryland Geological Survey, U. S. National Museum. 



