This proposal assumes that 'otwór potyliczny czaszki' refers to either 'otwór potyliczny wielki' or 'otwór wielki', and that the English term for this structure in humans is also used in the case of the species of interest.

Some Polish references for this structure:

aneksy.pwn.pl/biologia/1.php?id=1474327
http://aneksy.pwn.pl/biologia/1474328_1.html
http://pl.wikipedia.org/wiki/Kość_potyliczna
http://pl.wikipedia.org/wiki/Otwór_wielki

Compare these English references:

http://en.wikipedia.org/wiki/Occipital_bone
http://en.wikipedia.org/wiki/Foramen_magnum
http://education.yahoo.com/reference/gray/subjects/subject/3...

However, I am not an expert and it seems that there is at least one other opening in this bone, the hypoglossal canal. References for that:

http://en.wikipedia.org/wiki/Foramina_of_the_skull
http://en.wikipedia.org/wiki/Hypoglossal_canal
http://en.wikipedia.org/wiki/Hypoglossal_nerve
http://pl.wikipedia.org/wiki/Nerw_podjęzykowy