Ammonites with serrated keels (Anyalthei), exemplified by A. spinatus and margaritatus, abound in the middle and upper lies, and again in the oolites (e. g., A. cordatus and excavatus). They are succeeded by the Bothomagenses in the chalk— thick Ammonites with a line of tubercles in the place of the keel.
Ammonites with channelled backs (Colliciali) are repre sented in the lias (A. anguliferus), inferior oolites (A. Parkin soni), and middle oolite (A. anceps), and in the cretaceous strata by numerous species (e. g., A. 8e970.4243, lautu.s, and les catus), remarkable for their elegance.
Of the species with backs more or less squared, armatus and capricorwus occur in the lias, athleta and perarmatus in the Oxfordian. But the oolitic forms which have the back square, and ornamented with two rows of spines when young, like Goweri, Duncani (fig. 21, 1), and Jason, become rounded and unarmed in their old age.
Round-backed Ammonites abound in the lies and oolites. The snake-like annulatus, the spine-bearing coronatus, and fimbriatus with its ornamented fringes, have been regarded as types of small groups. A more important division (Ideal) is distinguished by nearly smooth whirls, constrictions recur ring at regular intervals. These are seen in A. tatricue, and others related to Heterophyllus ; in many neocomian Ammo nites, and in A. planula,tus of the lower chalk.
These constrictions, often accompanied by a prominent rib, undoubtedly indicate periods of rest, when the Ammo nite ceased for a while to grow. They may be traced in species belonging to other groups, as well, e.g., in biplex and triplieatus, as in the Ligati; but most frequently all indications are obli terated by subsequent growth. It has been a question whether the lateral processes of Ammonites Duncani (fig. 21, z), are
formed and removed periodically, or whether they are pecu liar to the adults, and mark the close of their outward growth. The first conclusion is more probable from analogy ; and they are commonly found with small and apparently young shells, but not (any more than the lateral spines of the living Argo naut) in those of adult size and condition.
It was remarked by the elder Sowerby that Ammonites were most beautiful when of middle growth, the ornamental characters being less developed in the young, and lost in the adult. The ribs and spines, and even the keel or furrow of the back disappear, in many instances, from the body-whirl of the full-grown shell.
Varieties of form, such as marked the paheozoic Nautilicke, are met with in the Ammondlicke, chiefly towards the close of their reign. The Baculite (fig. 21, 4), with its straight shell, is characteristic of the upper chalk ; and the Tut-rime, which is spiral, and usually a left-handed spiral, abounds in the lowest beds of the same formation. In Hamite8 the shell is straight, returning upon itself after a certain space, and forming a simple or complex hook. In Ptychoceras these limbs of the hook-like shell are in close contact. The Toxoceras is curved like a bow ; in Crioceras the discoidal whirls are separate ; and in Scaphites (including Ancyloceras) the shell, at first compact like an Ammonite, or open-whirled like Crioceras,lengthens out finally, and returns upon itself like the crozier of the Hamite. Hell coceras, again, connects the last with the Turrilite by its elevated spire terminating in a prolonged crozier.
Of these forms, Ancyloceras alone is found in the oolites ; all the rest are cretaceous ; and most abound in the alpine districts of the south of France.