Synapsids vs. Diapsids??

[sean.kerns@sdrc.com (Snake)]

The episode of Paleoworld last night focused on the Dimetrodon, and
its role as fore-runner of madern warm-blooded animals; interesting
show.
What I found confusing though, being new to paleontology, was the
distinction made between the two groups which the show said split
from one ancestor: The synapsids, which the show says included 
protomammals, and all of the current mammals. The other groupw
was diapsids, which included dinos and reptiles and, I owuld
guess birds?
The term "diapsid" was defined as meaning "two holes", and they 
showed a skull of some kinf which had two holes which the jaw 
muscles apparently squeeze through whan the animal bites. The 
problem with this was that this method limits the size of the
brain.
They then defined "synapsid" as meaning "one hole, and showed
how this allowed bigger brain development to occur, and all that
goos stuff. As an example, they showed a human skull, and darned
if it didn't have a hole on each side of the head... Now one hole
plus one hole = two holes, right? I found this kind of confusing
and contradictory, since it seems then that both synapsids and
diapsids have two holes. Is the difference then in the location
of the holes? It looked to me like the difference was that 
synapsids have them on the sides of the head, where they don't 
get in the way of the brain, whereas diapsids have them at the 
back of the skull, so that they stretch across the brain. 
Can someone help clear this up for me? 
Thanks.
Sean
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|    Sean R. "Snake" Kerns              e-mail: sean.kerns@sdrc.com |  
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|    Structural Dynamics Research Corporation    '79 AQHA           | 
|    These opinions aren't SDRC's...  They may not even be MINE...  |
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