Marylaxd Geological Survey 445 



spicuously elongated. The latter variety is usually referred to M. Icsvis, 

 but there is a character by which M. araiata is readily distinguished from 

 that species, as well as from M. hella and M. wJdtfieldi. This feature is 

 the extension of the deep and broad ventral sinus dorsally to meet the 

 median elevated portion of the dorsal valve. 



In Maryland M. arcvaia occurs typically, and associated with it at 

 Corriganville has been found one elongate form that was at first mistaken 

 for M. Icevis. It is, however, an arcuate form. It is this elongate, arcuate 

 variety that Meek and Worthen also described and figured as M. Icevis 

 Vanuxem ( ?). 



Length 2.8 cm. ; width 3 cm. 



This species occurs in profusion in the New Scotland limestone wherever 

 these strata are exposed. A few localities only are given in the following 

 paragraph : 



Occurrence. — Helderberg Formation, New Scotland Member. 

 Devil's Backbone, Corriganville, 21st Bridge, Maryland ; Cherry Eun, 

 West Virginia; Warren Point, Pennsylvania. Becraft Member. 

 Warren Point, Pennsylvania. 



Collections. — Maryland Geological Survey, New York State Museum, 

 U. S. National Museum. 



Meristella lata (Hall) 

 Plate LXXIV, Figs. 9-11 



Merista lata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 431, pi. ci, figs. 3a-3m, 



1861. 

 Meristella lata Hall and Clarke, 1893, ibidem, vol. viii, pt. ii, p. 78, pi. xliv, 



fig. 12. 



Description. — " Shell subovate or subquadrate, sometimes longitudi- 

 nally oval. Ventral valve longitudinally arcuate, gibbous in the middle, 

 the greatest convexity about halfway from beak to base, with a shallow 

 depression in front which extends upwards not more than one-third the 

 length of the valve, and is not seen in young specimens : beak elevated and 

 closely incurved over the opposite one. Dorsal valve abruptly elevated in 

 the middle, and somewhat flattened at the sides. 



