The grass snake is the largest species of British snake, and is identified by its olive green body and darkish spots or streaks on the flanks. However, colouration can vary. Grass snakes have a distinctive yellow and black collar behind the head and have round pupils. They can often be confused with the adder which has a more thick-set body and a distinct zigzag down its back, and it also has a vertically slit pupil. The adder is venomous, whereas the grass snake is not.