The fragments of jaws of Ornithosaurs from the Cambridge Greensand show a greater variety in size, form, and proportion than those from any other formation. The largest species were apparently the most singular in the shape of the snout, but after five-and-twenty years of collecting we still seem destined to know them only from fragments which, though extraordinary and suggestive, are tantalizing from their imperfect condition. It is impossible to tell whether the head was short or long, or what proportion it bore to the body, in the oase of these isolated specimens, but some of them, like the jaw-fragment which I am about to describe, show a power of tooth and massiveness of bone which could only have pertained to one of the most destructive and probably one of the largest of these animals.