Tetrabranchiata

Towards the conclusion of its growth the air-chambers of the Orthoceras frequently become shallower, and the siphuncle diminishes in size. These indications of changed or diminished energies are accompanied by a diminution or disappearance of the internal radiated structure in the last part of the siphuncle.

In

Orthocera8 bisiphonatum (Tretoceras, Salter) the body chamber is prolonged in the form of a marginal lobe, simulat ing a second siphuncle.

The genus

Bactrites of Sandberger also resembles an Ortho ceras with single-lobed sutures.

Ammonitidee.—In the second division or family of cham bered shells—those with lobed sutures and a marginal si phuncle—we find a similar series of forms, straight, spiral, and discoidal, but more varied and more highly ornamented.

One large genus

(Goniatites, fig. 20, to) is found in the Devonian, carboniferous, and triassic strata, and permanently resembles the youngest form of the Ammonites, having the su tures lobed but not foliated. They seldom exceed 6 inches in diameter and are usually very much smaller. The whirls are most frequently concealed to some extent, and often marked by cross furrows or " periodic mouths." The Ceratites are distinguished by having the lobes of the sutures serrated, while the intervening " saddles" (or curves directed towards the aperture) are simple. They are found in the tries of Europe, Thibet, and South America ; and again, though rarely, in the cretaceous strata of France and Syria—a circumstance quite anomalous in the history of the geological 1. Ceratites nodosus ; Muschelkalk, Bavaria.

1. Ammonites Duncani (spinosus, Sby.) ;

Oxford Clay, Wilts.

3. Turrilites.

3. Hamites.

4. Bacnlites.

6. Ancyloceras.

7. (Trigonellites or Aptychus), operculum of Ammonites.

8. (Rhyncholites hirundo), upper mandible of Nautilus arietis, Rein. Nut. chelkalk.

9. Lower mandible (Conchorynchus avirostris).

distribution of life. Many Ammonites, perhaps all, are like Ceratites when young.

A bisected specimen of the Ammonites obtusus, in the Hunterian collection (No. 188), shows well the extent of the last, or inhabited, chamber of the shell, and the effects of the influence of the animal matter of the decaying cephalopod upon the petrifactive processes after death. The liassic clay has penetrated as far as the retracted soft parts of the ammonite permitted : the decomposing mollusk had been partially replaced by crystals of silex, discoloured by the pigmental or carbonized parts of the animal. The silex, which has more

slowly infiltrated through the pores of the shell into the air-chambers, is of a much lighter colour. In the same col lection may be seen exemplifications of injury and repair of the shell. In No. 195, Ammonites Goliatkus, from " Oxford clay," a portion of the shell, at the period when it formed the dwelling-chamber, "had been broken away during the life time of the animal, and repaired by fresh nacreous material, wanting the ribbed structure of the originally formed shell.' The species of Ammonite exceed 500 ; and their range is co-extensive with that of the secondary rocks. They are found throughout Europe, and at the Cape, in Kamtschatka, Thibet, and S. India. They are absent from a large area of the United States, but are found in the cretaceous strata of New Jersey, Missouri, and the West Indian Islands ; also in Chili and Bogota.

The sections into which, for the sake of convenience, this extremely natural group has been broken up, are very ill-defined, and have no pretension to be considered sub generic. The group (called Co-ssiani) characterising the triassic period, is remarkable for many-lobed and elaborately foliated sutures—a circumstance more important because it is the oldest group, and associated with Ceratites and the last-surviving Goniatites and Orthoeerata. They abound in the "alpine limestone" of St. Cassian, and Hallstatt in Austria. A second group (Arietes), having the back keeled, with a furrow on each side of the keel, as in the great Ammo nites called Blicklandi and Coneybearei, mark the lias period ; they are less plentiful in the oolites, and are represented in the greensands by the Cristati, which are keeled, but not furrowed, and develope a "beak," or process, from the keel when adult. The Arietes pass by many intermediate forms into the Palmieri (e. g., A. serpentinus), also characteristic of the upper lias, and these are represented by a few quoit-shaped species (Disci), with sharp backs, in the oolites.