From the Thorndale, Dollarhide book -- There are 15 Texas counties in the 1870 census listed as NP, which the enumerators listed as "no population."[That sounds strange as I believe there had to be people in an area before it could be formed into a county.]
Hardeman Wilbarger Wichita Knox Baylor Archer Haskell Throckmorton Jones Taylor Callahan Runnels Concho Edwards Palo Pinto - was not enumerated separately and perhaps not at all.
Cass was renamed Davis in 1861 and changed back to Cass in 1871.
Tarrant Co. is the only county I know that is missing in the 1860 census.