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Alpheus Hyatt, that the initial chamber of the Nautiloidea corresponds not to the protoconch of the Ammonoids, but to the second chamber of the latter, and that there existed in the young Nautiloids a true initial chamber, a protoconch which was either uncalcified or deciduous. The shell of the living nautilus does not decide this question, as its early stages are unknown, and there is a little vacuity in the centre of the spirally coiled shell which may have been originally occupied by the true protoconch.

The septa in the Nautiloidea are generally concave towards the aperture of the shell, their curvature therefore directed backwards (fig. 1); in the Ammonoidea, on the other hand, the convexity is usually towards the aperture, the curvature therefore directed forwards. The lines along which the edges of the septa are united to the shell are known as “sutures,” and these in the Nautiloidea are simply curved or slightly lobed, whereas in the Ammonoidea they are folded in various degrees of complexity; the projections of the suture towards the mouth of the shell are called saddles, those in the opposite direction lobes. The siphuncle in the Nautilus pierces the centres of the septa, and in fossil Nautiloids it is usually central or sub-central. In a few cases it is marginal, and in that case may be external, i.e. ventral, or internal, i.e. dorsal. In Ammonoids the siphuncle is always marginal, and usually external. Its walls in the living Nautilus are strengthened by the deposit of calcareous granules, and in some fossil forms the wall is completely calcified. But this proper calcified wall is quite distinct from calcareous tubes surrounding the siphuncle, which are developed from the septa. In the pearly nautilus each septum is prolonged backwards at the point where it is pierced by the siphuncle, forming a shelly tube somewhat like the neck of a bottle. In many fossil forms these septal necks are continued from the septum from which they arise to the next, so that the siphuncle is enclosed in a complete secondary calcareous tube. In the majority of Nautiloids the septal necks are directed backwards, and they are said to be retrosiphonate. In the majority of the Ammonoids the septal necks are continued forwards from the septa to which they belong, and such forms are termed prosiphonate.

The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic periods. The Nautiloidea are the most ancient, appearing first in the Upper Cambrian, the genera being most numerous in the Palaeozoic period, and comparatively few surviving into the Secondary. On the other hand, the Ammonoidea are scarce in Palaeozoic formations, being represented in deposits earlier than the Carboniferous only by comparatively simple types, such as Clymenia and Goniatites. In the Secondary period Ammonoids were very abundant, both in genera and species and in individuals, and with few local exceptions none are known to have survived even to the commencement of the Tertiary. In the widest sense the genus Nautilus has existed since the Ordovician (Silurian) period, but the oldest types are not properly to be placed in the same genus as the existing form. Even with this qualification the genus is very ancient, shells very similar to those of the living Nautilus being found in the Upper Cretaceous.

It has been maintained by some zoologists that the Ammonoidea were Dibranchiate, though it would not follow from this that the shell was, therefore, internal. They are, however, generally classed with the Tetrabranchiata, and the absence of all evidence of the possession of an ink-sac is in favour of this view. There can be little doubt that they gave rise to the Dibranchiata.

About 2500 fossil species are included in the Nautiloidea, but only a few species of the genus Nautilus survive. Some of the fossil forms are very large, the shell reaching a length of 2 metres, or 6 ft. 6 in. Of the Ammonoidea more than 5000 species have been described, and some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in diameter.

Associated with various forms of Ammonoids there have been found peculiar horny or calcified plates, sometimes contained within the body-chamber of the shell, sometimes wholly detached. The most typical form of these structures has been named aptychus. It consists of two bilaterally symmetrical halves, of somewhat semicircular shape, and attached to one another by their straight inner margins, like a pair of doors. In some cases the aptychus is thin and horny, but more often it is thick and calcified, in which case the principal layer has a peculiar cellular structure. The surface may be smooth or sculptured, and one side is usually marked by concentric lines of growth. Another type is similar, except that the two halves are united in the middle line; bodies of this character are called synaptychus; they occur in the body-chamber of species of Scaphites. Another form called anaptychus consists of a thin horny undivided plate which is concentrically striated. This is associated with species of Ammonites and Goniatites.

Many theories have been proposed in explanation of these structures. According to Sir Richard Owen, the aptychus is an operculum developed in a part of the body corresponding to the hood of Nautilus. E. Ray Lankester suggested that the double plate was borne on the surface of the nidamental gland, with the form and sculpturing of which in Nautilus it closely agrees. On this view the aptychus would occur only in females. The most recent view is that these structures could not have been opercula because of their constant position inside the body-chamber, and that they were not external secretions at all, but a calcified internal cartilage situated at the base of the funnel.

Classification of Tetrabranchiata.—Cephalopoda in which the mantle is entirely enclosed by a multilocular siphunculated shell, which may or may not be coiled. Only the last compartment of the shell occupied by the body of the animal. Numerous pedal tentacles around the mouth, which are retractile within sheaths. Halves of the funnel not united. Two pairs of ctenidia, and two pairs of renal tubes without reno-pericardial apertures. Pericardium opens directly to exterior. Cephalic cartilage wholly ventral. Optic vesicles with apertures, without crystalline lens.

Sub-order 1. Nautiloidea.—Initial chamber not inflated, with dorso-ventral cicatrix at extremity.

 Fam. 1. Orthoceratidae. Shell straight or slightly curved, with a simple aperture, large terminal chamber and cylindrical siphuncle. Orthoceras, Silurian to Trias. Baltoceras, Silurian.

 Fam. 2. Actinoceratidae. Shell straight or slightly curved, with wide siphuncle contracted at level of septa. Actinoceras, Silurian to Carboniferous. Discosorus, Silurian. Huronia, Silurian. Loxoceras, Silurian to Carboniferous.

 Fam. 3. Endoceratidae. Shell straight, with wide margina siphuncle, necks produced into tubes fitting into one another. Endoceras, Silurian.

 Fam. 4. Gomphoceratidae. Shell globular, straight or arcuate, aperture contracted. Gomphoceras, Silurian. Phragmoceras, Silurian.

 Fam. 5. Ascoceratidae. Shell straight, ampulliform, summit truncate, terminal chamber extending nearly whole length of shell ventrally. Ascoceras, Silurian. Glossoceras, Silurian.

 Fam. 6. Poterioceratidae. Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa. Poterioceras, Silurian to Carboniferous. Streptoceras, Silurian.

 Fam. 7. Cyrtoceratidae. Shell slightly curved, aperture simple, siphuncle wide, septa approximated. Cyrtoceras, Devonian.

 Fam. 8. Lituitidae. Shell coiled in one plane with the terminal part uncoiled, aperture contracted. Lituites, Silurian. Ophidioceras, Silurian.

 Fam. 9. Trochoceratidae. Shell helicoidally coiled, dextral or sinistral, the last whorl generally uncoiled. Trochoceras, Devonian. Adelphoceras, Devonian.

 Fam. 10. Nautilidae. Shell coiled in one plane, aperture wide and simple, siphuncle central. Nautilus, recent. Trocholites, Silurian. Gyroceras, Silurian to Carboniferous. Hercoceras, Silurian. Ptenoceras, Devonian. Discites, Carboniferous.

 Fam. 11. Bactritidae. Shell straight, conical, siphuncle narrow and marginal, necks long, infundibuliform, sutures undulating. Bactrites, Silurian and Devonian.

Sub-order 2. Ammonitoidea,—Initial chamber spheroidal; siphuncle narrow and simple; septa convex towards aperture; sutures complex.

Tribe 1. Retrosiphonata.—Siphuncular necks projecting behind the septa as in Nautiloidea. Sutures form simple undulations. Occur exclusively in Palaeozoic strata from Devonian upwards.

 Fam. 1. Goniatitidae. Shell nautiloid, with simple sutures and ventral siphuncle. Goniatites, Devonian and Carboniferous. Anarcestes, Devonian.

 Fam. 2. Clymeniidae. Shell nautiloid, with simple sutures, siphuncle dorsal, that is, internal. Clymenia, Upper Devonian.

Tribe 2. Prosiphonata.—Siphuncular necks projecting in front of the septa. Sutures form deeply indented lobes and saddles.

 Fam. 1. Arcestidae. Globular and smooth or nearly smooth, with reduced umbilicus, terminal chamber very deep, an aptychus present. Popanoceras, Permian. Cyclolobus, Permian, Arcestes, Trias. Lobites, Trias.

 Fam. 2. Tropitidae. Shells globular, but having radiating and tuberculated costae. Thalassoceras, Permian. Tropites, Trias. Sibirites, Trias.

 Fam. 3. Ceratitidae. Shells coiled, with a large umbilicus, terminal chamber short, sutures with simple saddles. Trachyceras, Upper Trias. Ceratites, Trias. Dinarites, Trias.

Some genera with helicoidal shells are related to these coiled forms, viz. Cochloceras, Trias; also some straight forms, e.g. Rhab-doccras, Trias.

 Fam. 4. Pinacoceratidae. Shell compressed, smooth, terminal chamber short, sutures very complicated, convex. Pinacoceras, Trias.

 Fam. 5. Phylloceratidae. Shell coiled, the whorls overlapping each other, sutures formed of numerous lobes and saddles. Phytloceras, Jurassic.

 Fam. 6. Lytoceratidae. Shell discoid, whorls loosely united or uncoiled, sutures deeply indented, but with only three saddles and lobes. Lytoceras, Jurassic and Cretaceous. Macroscaphites, Cretaceous. Reunites, Cretaceous. Ptychoeeras, Cretaceous. Turrilites, Cretaceous. Baculites, Cretaceous.

 Fam. 7. Ammonitidae. Shell coiled, with narrow whorls which do not embrace one another, aperture simple, a horny anaptychus present. Ammonites, Jurassic. Arietites, Jurassic. Aegoceras, Lias.

 Fam. 8. Harpoceratidae. Shell discord and flattened, with a carinated border, aperture provided with lateral projections,