130 Systematic Paleontology — Middle Devoxiax 



anterior to the foramen; foraincui narrow, directly in the lonoitudinal 

 axis of the sliell, or often a little oblique. Surface finely and evenly 

 striated by iine regular [concentric] elevated striae, distant from each 

 other more than twice their width. The apex of the dorsal valve is about 

 one-third, and sometimes less than one-third the length of the shell from 

 the posterior margin. Perforation of the ventral valve naiTOwly oval or 

 snblinear, about one-third the length of the shell from the posterior 

 margin, and extending towards the edge of the shell." Hall, 1867. 



Hall noted the close relationship of Discina media H. to D. lodiensis 

 (Van.) suggesting that perhaps it was "only a well-marked variety of 

 that species" (Pal. N. Y., vol. iv, p. 21) in M-hich Walcott concurred 

 (Mon. U. S. Geol. Surv., vol. viii, p. 113) and it is now given by 

 Schuchert as Orhiculoidea lodiensis media (Hall) (Bull. U. S. Geol. 

 Surv., No. 87, p. 279). Dr. J. M. Clarke considers Orhiculoidea media 

 and 0. lodiensis as probably the same species, the difference being due lo 

 different conditions of preservation. vSome of the ]\Iaryland specimens 

 were compared with the types in the New York State Museum and it was 

 found that the concentric striae were very similar to those on the specimen 

 represented by fig. 25, pi. 2, vol. iv, Pal. N. Y. ; but the Maryland 

 specimens are of considerably smaller size. They may also be compared 

 with specimens of Orhicidoidea lodiensis (Tan.) from the Genesee shale. 

 The variety media is apparently distinguished from the species 0. lodiensis 

 mainly by its generally larger size, coarser concentric striae which are 

 also farther apart, and absence of the faint radiating folds. 



Kindle states that this is an abundant fossil of the Onondaga fauna at 

 most localities from northeastern Pennsylvania to northern Virginia. 



Length, 6^-7 mm. ; width, 5-^-6 mm. 



Occurrence. — Eomney Formation, Onondaga Member. Twenty-first 

 Bridge; Williams Eoad, three and one half miles east of Cumberland. 

 Hamilton Member. East bank Evitts Creek below Wolfe Mill ; Ernst- 

 \ille. 



Collections. — Maryland Geological Survey; American ]\Iuseum of Nat- 

 ural History; New York State Museum. 



