58 



Here, as elsewhere, the dorsal valves of Acrothyra give no indica- 

 tions of value as to orientation, but the high, prostrate ventral valves 

 do. A bed in this assise gave the following result : 



Other fossils ORIENTATION of valves on four slabs, layers from one inch to half an 

 s a inch a P art > ia the Assise E ' 3 a - 



Orientation of 

 three genera 

 in Assise E. 

 36. 



The dorsal valves, on one slab, of Acrothyra are recorded to show 

 how little they were affected by orientation. These slabs show an 

 unusual number of oriented Obolus ; many of these were young valves. 

 In the next assise above, orientation is well marked, as shown below : 



ORIENTATION in five small slabs from Assise R 3 b. 



Here Acrothele exhibits orientation to an unusual degree ; but in 

 other cases its round valves are found buried in the mud in all atti- 



