From Warhammer 40k - Lexicanum
I really dont like the part with the Tyranids, where it says: and to this day the Ultramarines seek out and destory all Tyranid infestations they can find. It makes the Ultramarines look like the big ever seeking tyranid cleaners and the only chapter perfect enough for Tyranid fighting. All the things with Nemeses, dont make much sense too me. Because all things that seeks to do harm against Mankind is a nemeses of the Space marines.