`   ``To put these discoveries in perspective,`` Dr. Novacek said of the new site, ``consider that the total accumulation of Cretaceous mammal skulls collected over 70 years all over the Gobi amounts to probably something less than 100 skulls. It is extraordinary to consider a 10-day haul in one square mile of nearly 150 skulls.``   Thats not millions or billions like one editor said. And this was a good dig.  `