These two North American ticks represent the Argasidae “soft ticks” (Lt), and the Ixodidae “hard ticks” (Rt).

Soft ticks, Argasidae, are oval or pear-shaped, with mouth parts that are more easily seen from the ventral view. Ixodes or “hard ticks” possess a dorsal plate or scutum, and many of them can transmit Lyme disease to humans.

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