TETRABRANCHIATA.
(Nautikdd Cephakipoda.) Of the lower group of Cephalopods, possessing chambered shells similar to the pearly Nautili, there are 1400 extinct species, belonging to above 30 genera, while 3 or 4 species alone exist in modern seas. These fossils resemble the Nauti lus, and differ from the dibranchiate Spiry,la in the structure of their shell, which is composed of two layers, the outer por cellanous, the inner pearly ; whereas the Spirula—an internal shell—is entirely nacreous. They also agree with the Nauti lus in the relative capacity of their last chamber, which seems obviously lnrge enough to contain the whole animal. More over, it appears, from the position of the siphuncle and the form of the aperture, that these shells were revolutely spiral, or coiled over the back of the animal, and not involute like the Spirnia. No traces of fossil ink (sepia) or horny claws have been found associated with them, nor any indications of dense muscular tissue, even in the same matrix which has pre served so completely the mummy cuttlefish. By their form and size they were ill adapted for rapid locomotion, and must have depended for safety on the shelter afforded by their solid shell. The discoidal Ammonites attained a diameter approach ing 3 feet, and the straight-shelled Orthocerata were sometimes not less than 6 feet in length. These latter must have lived habitually in a position nearly vertical ; whilst the discoidal genera would creep over the sea-bed with their air-chambers above them, like a snail-shell reversed. The Ammonites ap pear to have been provided with an operculum, more secure than the " hood" of the Nautilus, composed, like it, of two elements, not, however, fibrous and confluent, but calcified and united by a straight suture. These opercula, which have been mistaken for bivalve shells, have a porous structure altogether peculiar, and are frequently sculptured on their outer convex surface ; whilst their concavity exhibits only lines of growth (fig. 21, 7). Special forms are associated, in all localities, with particular species of ammonite ; and their size is adapted exactly to the specimens in which they are found.
Calcareous mandibles occur in all the secondary strata, but not, hitherto, in such numbers or circumstances as to imply that they belonged to any other genus beside the true Nautilus. They are of two forms : those corresponding to the upper mandible (fig. 21, s) have been called " Rhyncholites" (Palceo teuthis and Rynehoteuthis of d'Orbigny) ; whilst the lower man dibles constitute the genus Conehorhynchus of De Blainville (fig. 21, 9). The arms of the extinct Tetrabranchs may have been organized like those of the Nautilus, but were probably less numerous in the genera with slender shells, and in those early forms with a small many-lobed aperture. The length of the body-chamber is greatest when its diameter is least ; and the prominent spines which ornament the exterior are par titioned off internally by a nacreous lamina, indicating con siderable motion of the animal in its shell. When the outer shell of the fossil is removed by decomposition or the ham mer, the margins of the internal septa (or partitions of the air-chambers) are exposed : these marginal lines are called " sutures." The chambered shells may be divided into two principal groups, viz., those with simple sutures, like the recent Nauti lus ; and those in which the margins of the septa are lobed and foliaceous, the Ammonites. In the former the siphuncle is central or internal (i.e., at the margin next the spire) ; in the latter it is external (i.e., at the back of the shell, but ventral as regards the animal). There are, however, Nautili with lobed sutures (Aturia, Bronn, fig. 19, 1) ; and some with an exter nal siphuncle (Cryptoceras, d'Orb.) And on the other hand, the sutures of the Ammonite are at first very slightly lobed, and become progressively more complex ; so that specimens of the same species have been referred to three genera—Goni atites, Ceratites, and Ammonites—according to their age.