2o6 



ROCHESTER ACADEMY OF SCIENCE. 



depth, in which the conditions were probably virtually estuarine. 

 Remains of the characteristic life of the Paleozoic are completely 

 absent. 



The ridges by which A. alleghaniensis is known are found on the 

 under surface of sandstone layers resting in shaly partings. They 

 vary in width from y^^ to 3/^ of an inch and in relief from almost noth- 

 ing to as much as 2 or 3 inches. Usually they disappear into the 

 sandstone at either end. In length they vary from an average of 5 or 



Fig. 3. — Diagram of base of a specimen of 4^ volutions, showing self penetrations. Natural 

 size. 



The oldest portion preserved is at a ; the direction of formation was from a to b, to, c to rf, 

 and so on. 



6 inches to 40 or 50 in rare cases. Their surface is marked by regular, 

 shallow, transverse corrugations. In addition to these, the best pre- 

 served material shows fine parallel or interfering wrinkles extending in 

 the same direction. Usually there is a median longitudinal depres- 

 sion. The ridges cross and cut one another at every conceivable 

 angle, sometimes forming flabellations. 



