510 Contributions to Invertebrate Palaeontology. 



Genus MERl STELLA Hall. 



Meristella laevis. 



Plate V, figs. 6 and 7. 



Atrypa hevis Vanuxem, 1842, Geol. Kept. 3d Dist. N. Y., p. 120, fig. 2. 

 3/erista IcEvis (Vanux.) Hall, 10th Rep. State Cab., p. 94. 

 Merista (= Meristella) Icevis (Vanux.) Hall, Pal. N. Y., vol. 3, p. 247, pi. 39, 

 fig. 3. 



Shell below a medium size, longitudinally ovate in form, and very ventri- 

 cose ; ventral valve much longer than tlie dorsal, with a strong incurved beak, 

 from which the shell constantly widens to below the middle of the length ; 

 body of the valve flattened along the centre in the upper part, and gradually 

 becoming more and more depressed until it becomes concave toward the front, 

 forming a very distinct mesial sinus ; front slightly prolonged and bent up- 

 ward. Dorsal valve very convex in the upper part, apjiroaching gibbous on 

 the umbo, the beak small but strongly incurved ; front of the valve truncate 

 or slightly emarginate to accommodate the front extension of the ventral, but 

 no distinctly defined mesial fold exists. Surface of the shell marked only by 

 numerous concentric lines of growth, some of which are strongly defined. 



The specimens of this species noticed from Ohio are smaller than 

 the usual size of individuals from New York, but present the usual 

 features of the species as shown on the specimens figured by Prof. 

 IliiU, on plate 39, Pal. N. Y., vol. 3, fig. 3, f and k, which is by far 

 the most common and characteristic form among those from that 

 State. The Ohio specimens are internal casts, and show the slit in 

 the dorsal valve caused by the removal of the median septum very 

 distinctly. The castS' of the ventral side show the characteristic 

 form of muscular impression, l)ut it is small and faintly marked. 



Formation and Locality. — In the hydraulic beds of the Lower 

 nelderberg group, at Greenfield, Ohio. 



Meristella bella. 



Plate V, figs. 8-10. 



Merista bella Hall, 10th Kept. State Cab., 1857, p. 92. 

 Meristella bella Hall, Pal. N. Y., vol. 3, p. 248, pi. xl, fig. 1. 



Sh<-ll rather below a medium size, somewhat oblate in form, at least as 

 wide as long, the narrowing of the beak giving an oblate appearance to the 

 shell. Valves usually ventricose and sometimes highly convex, generally a 

 little more full above than below the middle ; margins of the shell regularly 

 curved except near the beak, which is slightly projecting and moderately 

 incurved. Surface of the valves smooth, but each characterized by a sliglitly 

 impressed mesial sinus along the centre, more strongly marked on the ventral 

 than on the dorsal side, and which not unfrequently causes an emargination 

 of the front border of the shell. 



