Convert bytea to Float8

Can someone please answer what I hope to be an easy question? I am trying
to convert 8 bytes of a bytea into a float8.

In C# this code looks like:
xCoord = System.BitConverter.ToDouble(arrByte, cnt) ;

But in my plpgsql function, the following gives me an error:
xCoord := substring(arrByte from cnt for 8)::float8;

The error that I get is:
ERROR: 42846: cannot cast type bytea to double precision

I have also tried get_byte, but that seems to only be returning the single
byte and not all 8bytes for the float. Thanks in advance.
Lee Keel

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