Originally Posted by iSaAc24
If you completed the game and replay certain levels to kill a dinosaur with a certain weapon, you must complete the level for the kill to register correctly.
I missed the 1855 too. Mid-game, I went back, replayed level 2, got the kill, and quit. Achievement didn't unlock at the end of the game. Went back to level 2 one more time, and as soon as I made the 1855 kill, the achievement unlocked - didn't need to finish the level.