Ammonites, a genus of fossil shells allied to the nautilus. The fossils are in the form of a coil or of a ram's horn, and the name is given to them from their resemblance to the horns upon the statues of Jupiter Amnion. They also resemble a snake in its coil, and are generally supposed by the common people to be petrified snakes. The animal that inhabited the shell was provided with air chambers, by means of which it could rise or sink in the water; and its shelly covering, necessarily very delicate in order to float, was made strong to bear the pressure at great depths by its tubular form and by the ribs or plates of shell that supported it within. From the lower rocks of the transition period up to the tertiary, the ammonite has been represented by many species. They abound especially in the oolite. They appear to have been very widely distributed over the ancient seas, the same fossil species being found in rocks of the same period in different quarters of the globe. They are common in the greensand formation in New Jersey, and far up the Missouri river. In Asia, at an elevation of 16,000 feet, in the Himalaya mountains, some of the same species have been found that are met with in England, and one of the same in the Maritime Alps, 9,000 feet above the sea.

They are so abundant in some parts of Burgundy that the roads are paved with them. In the chalk formation they are found of gigantic size, three and even four feet in diameter.

Ammonites Jason.

Ammonites Nodotianus.

Ammonites Cordiformis.

Ammonites Tornatus.