Fakiferi.—The back is narrow, acuminated to a sharp keel (no furrow on its sides) ; the ribs are elegantly and sigmoidally bent. The sutures differ from those of A rietes, the dorsal sinus D being much less deep, with diverging and not parallel sides • the sinus L is very much deeper, and there are three or four smaller ones, a a' a", near the inner edge of the whorls. The latter whorls usually embrace the preceding ones.
These Ammonites are numerous in the Upper Lies and Lower Oolite formations. A. Strangwaysii of Sowerby ia an example. (Ammonites Waleottii does not belong to this division.) Amalthei.—The back is generally acute and keeled, the keel generally crenated; the ribs generally a little sigraoidal ; the latter whorls embracing the preceding ones. The sutures are in general form much like those of the last division, but more richly laciuiated and foliaceous.
This group belongs to the Upper Lias and Oolitic formations. Ammonites amaltheus of Schlottheira (A. Stokcsii, Sowerby) is an example from the Lim.
Capricorni.—The back broad, without a keel. The ribs simple, straight, strong, and crossing the back. Inner whorls exposed. The sutures often approach to those of the A Hetes in respect of the sinus D ; but the posterior edge of D, L, and I range on the same line, and the undulations are all lower aud less foliaceous than in the Palmieri and Arnalthei. Ammonites planicostatus (Sowerby) is an example. The species are common in the Lias.
Planulati.—The back and sides rounded ; no keel ; the inner volutions exposed. The ribs are often divided over the dorsal region. The sutures are remarkably laciniated and complicated ; the sinus (L) extremely deep, and generally trifurcate.
The species occur commonly in the Lias and Oolitic formations. Ammonites communis (Sowerby) is an example from the Lias ; A. (Sowerby) from the Coralline Oolite.
Dorsal i.—The back is broad and not keeled ; the whorls often quadrate : the ribs are simple ou the sides, but divided over the Lack, and generally bear a tubercle at the point of division. Ammonites Darvi (Sowerby) is au example from the Lias.
Coronarii.—The back without a keel, usually broader than the sides ; the ribs are straight and simple on the sides, but divided into two, three, or more, as they cross the back, and the point of division is usually sharply tuberculate. The sutures resemble those of the
P/ardati. The species occur in the Oolites, as A. Humphresianus (Sow.); A. Gowerianus (Sow.); and iu the Lias, as A. Bechei (Sow.).
ilfacrocephali.—The back is without keel, and round and broad, and the umbilicus deep. The ribs are straight on the umbilical face and simple, but sometimes arched, and generally divided across the back. The sutures resemble those of the Planulati, but are somewhat differently proportioned to the dorsal and umbilical surfaces. The species occur in the Oolite and Chalk. Tho Ammonites sublevis (Sow.),'is a good example, from the Kelloways Rock.
A rmali.—The back without a keel, often broader than the aides ; ribs tuberculated on the sides. The inner whorls exposed. The sutures have the dorsal sinus (D) large and deep, the lateral sinus (L) widely removed from it and very deep, and somewhat trifurcate. Occurs in the Lias and Oolitc, and more plentifully in the Chalk. Ammonites Bakerige of Sowerby is an example.
Ornati.—The back flat or even hollow, narrow, and not keeled; the broad sides joining it at a right angle, marked in general by a row of small tubercles or the numerous fine ribs which cross the back and toward the inner edge unite in parcels to form acute or knotted ridges. (The old shells are often plain.) The sutures have the dorsal sinus (D) shallow, the lateral (L) deep. The species are almost confined to the Oxford Clay and Kelloways Rock : as A. Calloriensis (Sow.) ; A. Duncani (Sow). ; A. gemmed:Is (Phil.), &c. (Von Buch rightly separated from these the Dentati in his original memoir, though they have been injudiciously reunited again.) Dentati.—To this group we refer Ammonites splendens, A. leutus, A. dentatus, &c. of the Gault. The back is flat or concave, and margined by tubercles, or prominent ends of strong ribs, often united near the inner edge into tuberculated ridges, but not crossing the back. The sutures resemble in general form the preceding.
Fle.raosi.—The back narrow, with borders tuberculated or serrated by the terminations of the ribs, and in a young state with a tuber culated keel. The ribs are ridged or tubereled near tho inner edge. This group is quoted from the upper Oolitic and Chalk formations, and A. asper and A. flexuosus are examples..