These are two Ixodidae “American dog ticks”, Dermacentor variabilis, male is on the left, female is on the right.

The two major vectors of Rickettsia rickettsii, the bacteria responsible for Rocky Mountain spotted fever in the United States are the American dog tick, Dermacentor variabilis, and the Rocky Mountain wood tick D. andersoni.

Picture & text from CDC/PHIL. For more information on source & for information on color coding used above for different types of ticks, see Tick Pictures from CDC