188 NATURAL HISTORY. 



acquires its full growth, which is indicated by the body having receded for a less distance from the 

 penultimate septum before the formation of the last septum is begun." (Owen.) 



If we will only bear in mind this fact, that the animal of the Oyster and the Nautilus is 

 compelled by the constant, though almost imperceptible, growth of the mouth or border of the 

 shell to which it is attached by the margin of its mantle to move forward in its habitation, and that 

 its hinge-ligament and shell-muscles are absorbed behind and added to in front, to accommodate 

 themselves to the onward growth of the shell-border, of necessity, therefore, the animal cannot let go 

 its muscular or its siphuncular point of attachment to the old septal surface until the new one is made 

 ready ; hence the dipping down from layer to layer of the Oyster's shell-muscle ; hence also the curious 

 funnel-like tubes in Nautilus (Aturia) zic-zac. 



Although the Tetrabranchiate division of Cephalopods is only known by a single living genus, 

 the Pearly Nautilus, in Silurian times it was represented by thirty-four genera and above 1,600 

 species. The shell in all the animals of this division is, geometrically speaking, an extremely elongated 

 cone, either straight or variously folded and coiled. The Palaeozoic species, of which Orthoceras is the 

 type, had simple sutures, not complex, as is the case with the Secondary forms ; but they underwent 

 the same variations in curvature between straight in Orthoceras, bent on itself in Ascoceras, curved 

 in Cyrtoceras, spiral in Trococeras, discoidal in Gyroceras, produced discoidal in Lit nit es, and 

 involute in Nautilus. 



All the shells, as in Nautilus, are furnished with a siphuncle, sometimes having the appearance 

 of a string of beads, at others like the vertebral column of some higher animal. The largest British 

 Orthoceras does not exceed five to six feet in length, but they have been met with in America ten to 

 twelve feet long, and of proportionate bulk. From faint indications of colour-bands it seems probable 

 the shell was not an internal one, like the Belemnite, but rather like the modern Nautilus, external 

 to the soft parts of the animal. In the Goniatites and the Ammonites the septa of the shell have most 

 complex borders, which leave their impress upon the shell, and are called its sutures. They are 

 highly ornamental in the Ammonites, often resembling the foliage of plants in pattern. The 

 siphuncle, which is mostly central in Orthoceras, is marginal in Ammonites, running along the middle 

 of the keel of the shell. 



In the recent Nautilus the two " dorsal " arms are soldered together and expanded into a thick 

 hood or operculum of tough and rugose epidermis. In the fossil Ammonites calcareous matter is 

 added, thus forming a bi-lobed shelly operculum, or lid, which closely fits the mouth of the shell in 

 many of the species, and has been found in situ in a specimen of Ammonites subradiatus from the 

 Inferior Oolite of Dundry, near Bristol. One species from the Lias has a horny operculum. Thus 

 we have in Nautilus and Ammonites a perfect analogy to the Gasteropoda, in which there are Snails 

 without opercula, Snails with horny opercula, and others with shelly opercula. 



More than 700 species of Ammonites have been described, extending from the Carboniferous of 

 India to the Chalk, and they were of world-wide distribution. 



In the Chalk a number of Cephalopodous shells have the appearance of having become 

 " uncurled," and assumed fantastic forms of growth, as straight, folded in two, hooked, spiral, open 

 spiral, trumpet-shaped, discoidal in the young state, and uncurled in later life. 



The Tertiary Nautili differ but little from their modern representatives, save one form, named 

 Nautilus (Aturia) zic-zac, from the curious bent pattern of the septal partitions. In this form the 

 siphuncle is not continuous, but is made up of a series of rather thick funnel-shaped nacreous tubes 

 fitting one into another. This species is repeated, in a modified form, in the Secondary rocks, and 

 by what is believed to be a Palaeozoic representative in New Zealand. 



HENRY WOODWARD. 



