Mouth.-The terminal aperture of the shell. This is sometimes constricted (fig. 106) sometimes fully open, sometimes surrounded or flanked by remarkable expansions of the shell. The distance from the outer to the inner border of the

mouth is termed its height.

Aptychus.-A calcareous body like an operculum (fig. 105), formed of two plates, which resemble in outline the valves of a lamellibranch. These plates are often found detached; but during the life of the cephalopod they were brought together along their straight edges and occupied a position near the mouth of the shell, probably closing it as an operculum. In some cases the two plates are permanently united (Synaptychus). It has been sug

gested that those forms in which an aptychus is unknown may have possessed the structure in a horny condition.

Anaptychus.-A body resembling the aptychus, but formed of one plate only; found as thin dark lustrous impressions; probably a horny form of the operculum.

The shell-substance in cephalopods is aragonite, according to both Fuchs and Sorby. Mr. O. A. Schwarz (Geol. Mag., 1894, p. 457) shows that fragments of Nautilus have a specific gravity of 2.68, whence he infers that the material is calcite. But a considerable amount of organic matter is set free when such fragments are dissolved in acid, and is perhaps responsible for the low figures obtained on examining modern Nautili. The Ammonites, in their present condition, consist of calcite, as also do their aptychi and the guards of Belemnites. The view that the original material of Ammonites was aragonite is supported by the fact that whole beds of aptychi are known from which the Ammonites have been dissolved away. An aptychus is sometimes found lying surrounded by a mere impression of the spiral shell. In any case, it is probable that some structural difference accounts for this difference in resisting power.

The ready destruction of the shell causes casts of cephalopods to be very common. The body-chamber, and many or all of the interseptal chambers, may become infilled with crystalline calcite; the body-chamber is, however, often filled up with mud. When the shell decays, the casts thus formed of the successive chambers generally cohere, and the form of the shell is retained. But sometimes the sutures have become represented only by curving interspaces, and the cast is divided up by them into detachable

blocks. The extreme folding of the sutures in the Ammonites may cause these separate casts of the chambers to remain interlocked with one another and yet to possess a certain amount of freedom, so that they can be moved about on one another when taken in the fingers.

The

Cephalopod shells are composed of two layers, the outer one more opaque, the inner lamellar, thicker, and nacreous. septa appear to consist only of the nacreous layer. All the cephalopoda are Marine.

A. NAUTILOidea.

By analogy with the living Pearly Nautilus, the animals of the genera here placed are believed to have been tetrabranchiate, i.e., to have possessed four branchiæ. The shell is not (as in some dibranchiates) included in the body of the animal. It is straight, curved, or coiled, with a mouth of various form. The septa are, in typical examples, very simply curved, concave towards the anterior side of the shell, and forming suture-lines with, at most, very simple lobes. The shell, with the exception of the rare genera Bathmoceras and Nothoceras, is retrosiphonate; and the siphuncle stands away from the bounding wall of the shell, piercing the septa sometimes in their centre.

The surface of the shell is only plainly ornamented, if at all. Nautilus. Shell coiled in one plane, involute, but sometimes with a small umbilicus. Mouth not contracted, commonly rather high. Body-chamber large. Suture-lines forming a simple curve, or only slightly lobed. Siphuncle almost central. Surface smooth; very rarely with grooves or ridges. (Example:Nautilus pompilius.)

Trias to Recent. Nautilus is the only tetrabranchiate genus living at the present day.

Note.-The generic name "Nautilus" is now restricted as above. The older forms are widely umbilicated, and have often a perforation at the centre. Barrandeoceras (Ordovician and Gotlandian) and Trocholites (Ordovician) are especially interesting early representatives.

Discites. Shell laterally compressed, with broad shallow umbilicus, all the whorls being exposed. Small perforation at centre. Whorls four-sided in cross-section, sometimes with a groove on outer side. Suture-line forming a very simple curve. Surface with mere transverse growth-lines or delicate longitudinal ribs. (Example:-Discites mutabilis).

Carboniferous.

Lituites. Ally of Nautilus, but commencing with a small umbilicated (or even evolute) coil, and then continuing as a straight form, often of considerable length. Mouth often constricted, with a deep notch on the outer side. Siphuncle nearer inner side.

Ordovician and Gotlandian.

Orthoceras. Shell straight like a long cone; commonly circular in cross-section. Mouth not contracted. Body-chamber long. Septa simply curved, concave forwards. Suture-lines unlobed, or at most with very feeble foldings. Siphuncle central, or nearer to the margin; simple in character, but sometimes expanded laterally in each chamber. Surface of shell smooth, or simply ribbed.

Cambrian to Trias. Most abundant in Gotlandian.

Actinoceras.-Shell at times very large; often referred to Orthoceras. Like Orthoceras; but the siphuncle (endosiphon) is included in another much larger tube, which is expanded between the septa, forming a series of oblate spheroids, and at times as wide as half the shell. Delicate canals radiate from the siphuncle to the outer tube, and open into the interseptal chambers. The outer tube is frequently contracted internally by the development of obstructions of calcareous and organic material, deposited on its inner wall; these eventually form an annular thickening, which greatly reduces the tube. additions sometimes become dissolved away after the central tube has been infilled with mud; hence the primary wider hollow becomes restored, but a solid rod-like cast runs down its centre.

These

Cambrian to Carboniferous. Gomphoceras (fig. 106). Pearshaped, the shell having a straight or nearly straight axis; it commences as a wide cone, and finally closes over towards the mouth. Mouth much constricted, forming merely a T-shaped slit, the upright line of which is regarded as ventral, the cross-piece being dorsal. Septa and sutures simply concave. Siphuncle as in Orthoceras, varying in position in different forms. Surface smooth, or only finely striated. Gotlandian. Perhaps Ordovician; the fusiform later types are

probably Poterioceras. (Foord, Catalogue of Fossil Cephalopoda, pt. i. p. 215.)

Poterioceras.-Shell smooth, slightly curved, inflated in middle portion, and then again contracted. Mouth elliptical, not contracted. Siphuncle nearer to convex side, and inflated between the septa. Septa oblique to axis of shell.

Ordovician to Carboniferous.

Cyrtoceras. Shell like a curved and rather rapidly expanding Orthoceras. Cross-section generally oval. Mouth unconstricted. Septa and suture-lines simply concave. Siphuncle as in Orthoceras, but almost always near the convex side. Surface smooth in ordinary species, or only lightly striated.

Cambrian to Permian. Especially Gotlandian.

Phragmoceras. Many of the species placed under this genus have been transferred to Gomphoceras, Poterioceras, &c. Phragmoceras proper is distinguished from Gomphoceras by the curved shell, which at times even shows a trace of evolute coiling; and from Cyrtoceras by the constricted and T-shaped mouth. Ordovician and Gotlandian.

B. AMMONOIDEA.

The members of this group have often been closely connected with the Nautiloidea under the title of Tetrabranchiata. However, from the globular, and not conical, form of the initial chamber of the Ammonites (which resembles the first stage of the chambered body in Belemnites, Spirula, &c.) some zoologists place them as dibranchiates. The group attains its fullest development in the Mesozoic era, where it terminates.

Exceptions to the typical mode of coiling of the shell are probably rarer among the Ammonoidea than among the Nautiloidea. Straight or evolute turreted forms come in most numerously in the later Mesozoic deposits. The margin about the mouth differs from that of the Nautiloidea in very often bearing a broad or spine-like prolongation on the convex side, and sometimes ear-like processes on its lateral margins. No such remarkable constriction of the mouth occurs, however, as in Gomphoceras and its allies, the expanded processes in the Ammonites pointing fairly forwards. The body-chamber is on the whole larger than in the Nautiloidea; but it must be borne in mind that this final portion and the mouth of the shell, being unsupported by septa, are comparatively rarely preserved.

The suture-lines are typically more complex than in the

Nautiloidea, and the amount of folding increases from the first septum to the later ones. Considerable attention has been paid to the form of the suture-lines, and their common course is as follows (fig. 107):-A lobe, the External lobe (or "ventral lobe"), occurs on the convex side of the shell, and is sometimes divided into two by a small såddle (fig. 113). On either side of this lobe comes a saddle (the External saddle); then a lobe (the first lateral lobe); then the first lateral saddle, the second lateral lobe, the second lateral saddle, and perhaps still further lobes and saddles, which are styled auxiliary. On the concave side of the whorl, where the two halves of the suture-line again meet, there occurs an unpaired Internal lobe. The last auxiliary saddle, occurring just above this lobe, is sometimes called the Internal saddle.

The siphuncle of the ammonoids, with the exception of the one genus Clymenia, runs along the convex side of the shell. The group of the Ammonites is prosiphonate.*

The surface of the shell is often, and particularly in the later types, highly ornamented with ribs and knots, which are independent of the suture-lines (figs. 113 and 114), the latter being visible only upon worn specimens and casts.

At

times, as in Gault specimens from many localities, the inner and thicker nacreous layer is alone preserved, and the whole surface of the shell has a brilliant pearly iridescence.

el

Ill

ils.

il.

a

Fig. 107.-Suture-line of

an Ammonite (Harpoceras). The mouth lies to the right. el, External lobe, bisected by a median saddle on the keel. e.s, External saddle. 1. ll, 1st lateral lobe. 1. ls, 1st lateral saddle. 2. U, 2nd lateral lobe. 2. ls, 2nd lateral saddle. a, u, Auxiliary lobes and saddles. i.l, unseen Internal lobe.

Finally, the bodies known as aptychi (fig. 105) or anaptychi are found associated with so large a number of ammonoid genera a further point of difference between this Nautiloidea.

that they form group and the

*Forms of Ammonite occur in which the earlier whorls are retrosiphonate; in one or two septa following on these the septal neck projects on both sides; and finally the shell becomes purely prosiphonate. But the broad classification of the Ammonoidea by the direction of the septal neck in adult forms seems well founded, since the older genera are so persistently retrosiphonate, while those of Mesozoic times are prosiphonate.