what is the correct for xpath selector for xsi:type

Hi

I am trying use libxml2 to parse the following xml. 

<?xml version="1.0" encoding="UTF-8"?>
<zoo:cageRequest
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:zoo="http://www.example.org/Zoo"
    xsi:schemaLocation="http://www.example.org/Zoo ../XSD/ZooRequest.xsd ">
  <animal xsi:type="zoo:Fish">
      <name>tuna</name>
      <numberOfFins>4</numberOfFins>
  </animal>
</zoo:cageRequest>

I need to do some special processing based on the value of xsi:type.  I eventual get a a xmlNodePtr that points to   

<animal xsi:type="zoo:Fish">
      <name>tuna</name>
      <numberOfFins>4</numberOfFins>
  </animal>

I can not seem to select the attribute. "@*" works but is not going to work long term.

"@xsi:type" and "@type" did not work

thanks

Andy