Before commencing a descriptive catalogue of these Ammonites, it may be as well to give some idea of the various beds from which they come. This has been done before; but, various mistakes having crept in and new researches having given us fresh information, I deem it worth while to reintroduce the subject. The beds under consideration begin with the “Sands” or “Passage-beds,” also called Midford Sands. They are from about 100 to 150 feet, perhaps more, in thickness, with interpolated layers of comminuted shells about a foot or so thick, occurring at intervals of from 4 to 10 feet. These layers contain a large conglomeration of broken shells, so that good specimens are scarce. I have obtained from them a Rhynchonella which is probably R.cynocephala (Richard.), besides Harpoceras Moorei (Lycett), and several species of Trigonia (clavellated and costate), Lima, Astarte , &c.; and I am led to suppose that these sands are probably equivalent to the lower part of the Cynocephala stage of Lycett, in the Cotteswold Hills. Above these sands comes the stone of the Inferior Oolite. This stone changes at certain levels, both in composition, colour, and hardness. The Ammonites do not occur all in one bed, but this Inferior Oolite stone can be very well divided into four zones, which are extremely well marked, but vary very greatly in thickness at different localities; and it is probably this variation in thickness, and sometimes almost complete absence of a zone, that has led to very much confusion. At