24

PALAEONTOLOGY.

shorter than the upper lateral: the auxiliary lobes are obliquely directed backwardstowards the umbilicus in a marked manner. A group which would be peculiar tothe Oolitic period, were it not that three species of the lower Chalk, of which thelobes are only imperfectly known, are provisionally included in it.

21. Coronarii: a group which specially characterizes the lower Oolite. It is dis-tinguished from the Planulati by having a knob or tubercle at the point where theribs or striae bifurcate; whorls elevated; septa composed of lobes divided into un-equal and of swells formed of equal parts; the dorsal lobe shorter than the upperlateral; the auxiliary lobes oblique: the upper lateral lobe is outside of and the in-ferior lateral lobe within the tubercles.

22. Macrocephali: shell analogous in form, ribs or striae, to that of the groupCoronarii, with this difference, that it is often more swollen, and the tubercle, insteadof being placed about the centre of the breadth of the whorl, is nearer the umbilicus,so that both the upper and lower lateral lobes are outside the tubercle, and not onewithin and the other without. The most swollen species proceed from the Ooliticstrata, but the group itself extends into the Cretaceous.

23. Fimbriati (D’Orb.) : shell discoidal, composed of cylindrical whorls which aregenerally contiguous, without in any manner covering each other, and are either smoothor transversely marked at intervals by projecting ribs or by grooves which were theformer maTgin of the mouth; mouth circular; septa symmetrical, formed of lobes andswells divided into equal parts, and always enlarged at their extremity and narrowedat their base; dorsal lobe often the longest. This group, so well characterized, isfound both in the lower portions of the Oolitic and of the Cretaceous periods : thegreater number of species belong to the lower greensand, or base of the Cretaceous system.

It has appeared desirable to give these ample details on the highly important genusAmmonites, as it will often be necessary, in geological researches, to seek the meansof identification of strata in the several groups into which it is divisible, as in many casesthose mineral differences which may assist in such inquiries, and are in some locali-ties or countries well marked, disappear entirely in others. With the Chalk, Ammo-nites disappear, the upper section of the white chalk being deprived of them. Inthis interesting and varied family, therefore, of the most highly organized class of Mol-lusca (the Cephalopoda ), each successive fauna of the earth’s surface possessed charac-teristic representatives, commencing with the Goniatites of the Carboniferous strata;and so beautifully, nay, so mysteriously has this great law of creative intelligencebeen manifested in the distribution of organic bodies, that D’Orbigny recognizeseven characteristic species in his three divisions of the Cretaceous system, assigning

75 species to the Neocomien, or Lower Greensand.

42 do. to the Gault.

27 do. to the Chalk, including the upper Greensand and true Chalk.

And so striking is this limitation of the species to particular faunae or epochs, that hethus remarks upon it: “ After comparing thousands of these Ammonites from all partsof France , I have come to this important result, that it is not merely some species, ashas been hitherto supposed, which are characteristic, but that all the species ofAmmonites, without exception, are characteristic, and that all indicate with certaintythe strata to which they belong, when the application of their evidence is made witha critical knowledge of the species.”

To the Ammonites may be added other remarkable genera of Cephalopoda whichare peculiar to the Cretaceous period:

Crioceras, differing from Ammonites in having the whorls, though in one plane,perfectly separate from each other, and not contiguous or enveloping. The lobes of