Maryland Geological Survey 363 



the two forms hold different horizons, in which case both names shoiihl 

 stand. Both species, but especially U. nbruptus, resemble species of 

 Eatonia of the E. sinvata type to such an extent that for a time the writer 

 referred specimens to this genus. The ventral muscular area, however, 

 has the character of Uncinulus. 



Length 19 mm. ; width 21 mm. 



Occurrence. — Helderberg Formation, Xew Scotland Member. 

 Devil's Backbone, Corriganville, 21st Bridge, Maryland ; Iveyser, West 

 Virginia,. Becraft Member. Ernstville, Xorth Mountain, Maryland; 

 Cherry Bun, West Virginia. 



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



Uncinulus abruptus (Hall) 

 PlateLXlV, Figs. 11, 12 



Rhynchonella abrupta Hall, 1857, Tenth Ann. Rept. N. Y. State Cab. Nat. 



Hist, p. 68, fig. 1. 

 Rhrjnchonella abrupta Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 228, pi. 



xxxi, fig. 3. 

 Uncinulus abruptus Hall and Clarke, 1893, ibidem, vol. vili, pt. ii, p. 199, 



pi. Iviii, figs. 15-21. 



Description. — " Shell transversely oval, subpentagonal : ventral valve 

 depressed convex, very abruptly deflected towards the opposite valve on 

 the lateral margins ; beak small, depressed on the outside and subangular 

 along its lateral slopes, closely incurved over that of the opposite valve : 

 dorsal valve much the larger, very prominent in front, obliquely declin- 

 ing towards the beak ; beak depressed, incurved. Surface ornamented by 

 from twenty-five to thirty-three simple subangular plications, seven or 

 eight of which are elevated towards the front of the dorsal valve into a 

 somewhat distinct mesial prominence; and from six to eight of those on 

 the middle of the ventral valve are depressed so as to form a broad un- 

 defined sinus, which scarcely extends beyond the center of the valve 

 towards the beak, but is prolonged in front, and abruptly bent upwards 

 nearly at right angles to the dorsal valve into a distinct linguiform ex- 

 tension. The plications are marked in front by the usual longitudinal 

 depressed line along the center of each, and extremely fine regular zigzag 



