Kilayim, Chapter 1, Mishnah 2. This mishnah, like Mishnah 1, lists pairs of items that one might have thought are different species, but in fact, as far as the laws of kilayim are concerned, they are considered the same species and therefore are not kilayim with one another.
The pairs that are not kilayim with one another:
"HaKishut vehaMelafefon" - the kishut is a type of cucumber, and the melafefon is the common cucumber. They are not kilayim with one another. However, Rabbi Yehudah says: they are kilayim - in his opinion these are two different species, and therefore they are kilayim with one another.
"HaChazeret vechazeret galim" - lettuce and wild lettuce, which tends to grow in mountainous areas. Both are one species.
"Ha'ulshin ve'ulshei sadeh" - cultivated garden endives as opposed to endives that grow in the field and are not cultivated.
"HaCharia vecharia'ei sadeh" and "hakerishin vekerishei sadeh" - in each pair: the domesticated species as opposed to the undomesticated one. They are not kilayim with one another, but rather one species.
"HaKusbar vekusbar sadeh" - coriander and wild coriander.
"HaChardal vechardal Mitzri" - mustard seed and Egyptian mustard seed, which are considered one species.
"Udela'at haMitzrit veharemutzah" - the dela'at haMitzrit is the Egyptian gourd. The remutzah is a bitter type of gourd, and the widespread practice was to sweeten it by placing it in hot ashes, called remetz in Hebrew, and hence its name. Even though one is an Egyptian gourd and the other a bitter gourd that requires sweetening, both are considered the same species.
"VehaPol haMitzri vehecharuv" - the Egyptian bean together with the charuv. The word "charuv" can be understood as the carob itself, but there is also a carob that has beans inside it. These two as well are not kilayim with one another.
In summary: the members of each and every one of these pairs are not considered kilayim with one another, since they are one species. Mixing between the different pairs, however, would of course be considered kilayim.