444 Systematic Paleontology 



M. prcenuntia is undoubtedly derived from the Keyser W. ( ?) prosseri, 

 which also has the linear dorsal sinus and subangulated ventral sinus. 

 The latter, however, is always a smaller shell, is never arcuated dorsally, 

 and there are no rudimentary plications. From the evidence at hand, 

 it would seem that M. whitfieldi varied in two main directions, first 

 towards elongation, rotundity, and reduction of the ventral sinus, result- 

 ing in M. Icevis and M. hella. The other line tended to accentuate the 

 ventral sinus, to obliteration of the linear dorsal sinus by arcuation of 

 this region, and to a more subquadrate outline of the shell, resulting in 

 the M. arcuata group. 



Length 1.8 cm.; width 1.8 cm. 



Occurrenae. — Helderberg Formation, Keyser Member. Abundant. 

 Keyser, West Virginia; Dawson, Corriganville, Tonoloway, Maryland. 



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



Meristella arcuata (Hall) 

 Plate LXXIV, Figs. 5-8 



Merista arcuata Hall, 1857, Tenth Ann. Rept. N. Y. State Cab. Nat. Hist., 



p. 95, figs. 1-4. 

 Merista arcuata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 249, pi. xli, figs. 



la-1/, 1861. 

 Meristella arcuata Hall, 1867, ibidem, vol. iv, p. 298, text figs. 1, 2. 

 ? Merista Iwvis ? Meek and Worthen, 1868, Geol. Surv. 111., vol. iii, p. 376. 



pi. vii, figs. 8a-8c. 

 Meristella arcuata Hall and Clarke, 1893, Nat. Hist. N. Y., Pal., vol. viii, pt. ii, 



pi. xliii, figs. 1, 2; pi. xliv, fig. 5. 



Description. — " Shell broad-ovate, sometimes transversely oval. Ven- 

 tral valve longitudinally arcuate, gibbous in the central and umbonal 

 region, having in front a shallow rounded depression scarcely reaching tlie 

 middle of the valve ; front margin (in old specimens) elevated and fitting 

 into the broad rounded sinus of the opposite valve. Dorsal valve often 

 abruptly elevated along the middle and sloping laterally, having no dis- 

 tinct mesial fold; beak incurved. Surface smooth, or marked by faint 

 concentric lines and occasional stronger wrinkles of growth." Hall, 1857. 



This species, as a rule, is subquadrate in outline, but sometimes is con- 



