GEOLOGY AND PALEONTOLOGY OF THE SCHOHARIE VALLEY 119 



Brachial valve 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 umbonal portion but lower down including equidistant 

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

 radiating lines are visible. 



A young specimen aV 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 in it rather strong. 



A comparison with specimens from the Coeymans 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 COEYMANS 

 VARIETY VARIETY 



Length 5.5 nun 6.5 id m 



Width 4.5 nuD t> mm 



Depth 3.7 mm 3.5±mm 



The Rondout and Manlius are the cavern formations of the 

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

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

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



