442 Systematic Paleontology 



WhITFIELDELLA ( ?) MINUTA n. sp. 



Plate LXXIII, Figs. 41, 43 



Description. — Shell small to miniite, elongated in a direction from beak 

 to base, giving to the shell a somewhat subovate outline. Both valves 

 strongly convex, gibbous to somewhat ventricose, lateral and antero-lateral 

 margins regularly rounded, cardinal margins convex, but these curves are 

 sharper than the curves of the anterior margin. Ventral valve the longer, 

 convex, gibbous to subventricose. Greatest convexity just posterior to the 

 middle of the valve. Beak slightly incurved, prominent, pointed, extend- 

 ing beyond that of the dorsal valve. Dorsal valve regularly convex, 

 gibbous, greatest convexity in the umbonal region ; beak pointed, incurved, 

 filling the delthyrium of the ventral valve. Surface smooth or marked by 

 very faint obscure concentric lines of growth and on exfoliated individuals 

 fine silk-like striations radiate from the beak. No fold or sinus on either 

 valve. In some individuals the anterior portion is prolonged into a modi- 

 fied linguiform extension, while in other individuals the anterior margin 

 is rounded. 



Size of largest individual 3 mm. long and 3 mm. wide. 



This species is distinguished by its minute size. There is no fold or 

 sinus on either valve. 



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

 Backbone, Roundtop. 



Collection. — Marj^land Geological Survey. 



[Maynard.] 



Genus MERISTELLA HalP 

 The species of this genus are readily divided into two groups according 

 to the development of the fold and sinus. In the group with the oldest 

 representatives, which is also the one with the greatest number of species, 

 the ventral sinus becomes more and more accentuated in younger forma- 

 tions, while the anterior portion of this valve is tongue-shaped and may 



^ For an emended description of this genus, see Hall and Clarke, 1893, Nat. 

 Hist. N. Y., Pal., vol. viii, pt. ii, p. 73. 



