Datasheet

Page 22
Board is equipped with 8MHz crystal oscillator (X1) circuit
that provides external clock waveform to the microcontroller
CLKO and CLKI pins. This base frequency is suitable for further clock
multipliers and ideal for generation of necessary USB clock, which ensures
proper operation of bootloader and your custom USB-based applications.
The use of crystal in all other schematics is implied even if it is purposely left out, because of the schematics clarity.
5. Crystal oscillator
Figure 5-1:
Crystal oscillator
module
NOTE :