Maryland Geological Survey 449 



Genus MERISTA Suess 



MeRISTA TYPA (Hall) 



Plate LXXIV, Figs. 20-24 



Camarium typum Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 487, pi. xcva. 



figs. 2a, 3, 5, 6, 1861. 

 Camarium elongatum Hall, 1859, ibid., p. 488, pi. xcva, fig. 4. 

 Merista typum Hall, 1860, 13th Ann. Rept. N. Y. State Cab. Nat. Hist., p. 93, 



figs. 10-13. 

 Merista typa Hall and Clarke, 1893, Nat. Hist. N. Y., Pal., vol. viii, pt. ii, pi. 



xlii, figs. 7-12. 



Description. — " Shell short-oval or subelliptical, length but little 

 greater than the widths very ventricose or subglobose. Ventral valve much 

 the larger, extremely arcuate, the curvature from beak to base being some- 

 what more than half a circle; most gibbous on the umbo and near the 

 front, which is bent abruptly upwards, and produced into a broad rounded 

 extension: beak incurved, thin and pointed; umbonal slopes sharply 

 angular, subparallel to the cardinal margin, with a moderately broad, 

 smooth and slightly concave space between; foramen large, triangular, 

 reaching nearly to the extremity of the beak. Dorsal valve most gibbous 

 on the umbo, elevated in front into a prominent mesial fold; the sides 

 curved downwards, and produced to meet the receding edges of the oppo- 

 site valve: beak rather large, incurved beneath the opposite beak, and 

 apparently filling its foramen. The interior of the ventral valve has 

 a deep large rostral cavity, and a more or less highly arched trans- 

 verse septum, which, rising from beneath the rostral cavity, extends 

 to near the middle of the valve, and reaches about two-thirds across its 

 transverse diameter. From the bases of the dental plates, rise two diverg- 

 ing thickened elevated ridges, which extend to the sides of the arching 

 septum, and, uniting with it, gradually die out upon its surface. Surface 

 of the shell, when sufficiently well preserved, bears evidence of faint 

 radiating striae." Hall, 1859. 



Length 2.5 cm.; width 2.1 cm. 



Occurrence. — Helderberg Formation, Keyser Member. Devil's 

 Backbone, Cash Valley, Viaduct and Market Street Bridge Cumberland. 



Collection. — Maryland Geological Survey. 

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