st: covariance structure name?

[David Airey <david.airey@vanderbilt.edu>]

What kind of covariance structure is the following, that is, does it  
have name? Obviously, the correlations drop off for time points more  
separated (b1_b2 vs b1_b6), but also, the correlation changes over  
time (b1_b2 vs b5_b6). These correlations are of a behavioral measure  
every minute for 6 minutes in a couple hundred mice.

             |       b1       b2       b3       b4       b5       b6
-------------+------------------------------------------------------
          b1 |   1.0000
          b2 |   0.8297   1.0000
          b3 |   0.7096   0.7708   1.0000
          b4 |   0.5865   0.5950   0.7378   1.0000
          b5 |   0.5103   0.5177   0.6201   0.7111   1.0000
          b6 |   0.4095   0.4479   0.5271   0.5489   0.6815   1.0000

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