how to divide a large number of div blocks in smaller chunks

how to divide a large number of div blocks in smaller chunks

Hi

[div id="WhateverHolder"]

[div class="item"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item"][/div]

[/div]

that i need to have converted into

[div id="WhateverHolder"]

[div class="block"]
[div class="item first"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item last"][/div]
[/div]

[div class="block"
[div class="item first"][/div]
[div class="item"][/div]
[div class="item"][/div]
[div class="item last"][/div]
[/div]

[/div]

I have looked for a plugin that can do this but to no avail what is the easiest way to do  this in jQuery