44 ITEW TOEK STATE MUSEUM 



Meristella lentiformis, sp. nov. 



Plate 6, fig. 5-11 



1892. Meristdla, sp. nov., Clarke, op. cit. p. 414 



Meristella lenta Hall, is a diminutive species, wliicli lias 

 been reported only from the Oriskany sandstone at Cayuga and De 

 Cewville Ont.' It is of peculiar form, having the pedicle-valve 

 broadly concave in the pallial region and the brachial valve highly con- 

 vex medially, the sides sloping from this median prominence with a 

 gently concave surface. In respect to this form, which is an extreme 

 expression of a tendency frequently exhibited among the more equicon- 

 vex Meristellas, the species approaches the Meristella (Pen- 

 tagonia) unisulcata Conrad, of the Onondaga and Hamilton 

 beds. In the structure of the brachidium both of these shells have proved to 

 be genuine Meristellas. The most abundant representative of this 

 genus in the Becraft mountain Oriskany is a shell which nearly 

 approaches Mer. lenta in contour, with some noteworthy differences, 

 not the least being its considerable size. We may characterize the 

 species as follows: shell unequally convex; outline transversely oval; 

 pedicle-valve with short incurved beak and narrow cardinal slopes 

 bordered by obtuse cardinal ridges diverging from the beak. Umbo 

 slightly convex or flat, the surface sloping to the sides very gradually. A 

 median sinus starts at the umbo and rapidly broadens, producing a general 

 depression in the pallial region, which is rather sharply deflected. The 

 sinus is produced into a linguate extension at the anterior margin. 

 The general flatness of this valve is more marked in young shells, the 

 anterior deflection becoming prominent with the increase of age. The 

 brachial valve has a full beak curved into the delthyrium of the oppo- 

 site valve, and is elevated medially into a broad ridge-like concavity 

 terminating in a fold on the anterior margin. The lateral slopes are 

 gently concave. The surface of both valves is smooth or bears only 

 concentric growth lines. 



On the interior the apophyses and impressions are those characteriz- 



*For figures of this species see the original description, Paleontology of New York, 

 4:420, pi. 63, fig. 19-22 and also v. 8, pt 2, pi. 44, fig. 15-lS. 



