GEOLOGY AND PALEONTOLOGY OP THE SCHOHARIE VALLEY 119 



Brachial valye scarcely less arched than the pedicle valve, 

 with a median longitudinal flattening or faint depression on the 

 smooth portion. Anterior portion alone marked by a rather 

 pronounced median elevation which is gently grooved at the 

 center. Three rounded marginal plications occur on either side, 

 decreasing in size away from the center and becoming pro- 

 nounced and angular toward the front in the adult shell. 



Surface marked by fine lines of growth which are faint on the 

 smooth umbona] portion but lower down including equidistant 

 stronger ones, which give the surface a lamellose aspect. Faint 

 radiating lines are visible. 



A young specimen -5-0 inch long [fig. 27 c, d] has the plications 

 just appearing in the form of a wavy margin. The brachial 

 valve in this is less convex, rather flattened and marked by a 

 median depression. A slightly younger shell is perfectly smooth 

 except for the well developed concentric striae [fig. 27 a, b]. 



In adult individuals [fig. 27 f-h] the plications are strong and 

 angular at the front and the median depression profound with 

 the central plication: m it rather strong. 



A comparison with specimens from the Ooeymans limestone 

 shows this variety to be narrower, thicker, with a more sharply 

 depressed sinus, in which the fold is less pronounced, and with 

 more angular plications. The difference is shown by the follow- 

 ing measurements of two characteristic specimens. 



TRANSITION OOEYMANS 



VARIETY VARIETY 



Length 5.5 mm 6.5 mm 



Width 4.5 mm 6 mm 



Depth 3.7 mm 3.5± mm 



The Eondout and Manlius are the cavern formations of the 

 region, for in them are found Howes cave on the Cobleskill; 

 Ball's cave on Barton hill (though here the entrance is through 

 a fissure in the overlying Goeymans), Clark's or Gebhard's cave, 



