NAII TILUS, a genus of tetrabranehiate ecidialapoda (q.v.), extremely interesting as the existing representatives of an order of mollusks now redneed to a very few species, but of which the fossil remains attest the great abundance in former geological periods. The species of this genus are found only in the seas of warn' climates. One or more of them must have been known to Aristotle, as appeals from his description, which, how ever, is not minute. Yet it is but recently that they came under the observation of modern naturalists; and they were very imperfectly known, till a specimen, obtained by Dr. Bennett in a hay of the New Hebrides in 1829, was submitted to the examination of prof. Owen, and became the subject of a valuable memoir by him. The shell, indeed, lets long been common enough in collections, being plentifully found, entire or in frag ments, on many tropical shores;- but from the shell alone little could be learned concern ing the animal to which it belonged. The shell is spiral, the spire not at all elevated; and thus, in external form, resembles the shells of many species of snail; but internally it is camerated, or divided into chambers, by transverse curved partitions of shay mat ter. In a very young state this structure does not exist; but as the animal increases in size it deserts its first habitation,which then becomes an empty chamber, and so proceeds from one to another still larger, occupying the outermost only, but retaining a connection with all by means of a membranous tub,. (siI)1cunele) which passes through the center of each partition. The use of this connection is not known; but the most probable sup position is that the animal is enabled, by throwing air or souse kind of gas into the empty chambers of the shell, or by exhausting them of air, to change the total weight, so that it may rise or sink in the water at pleasure. It commonly inhabits the bottom of the sea; where it creeps about, probably like the gasteropods, by means of a large muscular disk with which the head is furnished; but it sometimes rises to the surface, and is to be seen floating there. Dr. Bennett states that the specimen which he fortunately captured attracted his attention, when thus floating, as an object resembling a dead tortoise-shell cat. The story of its spreading a sail is as fabulous as the similar story regarding the argonaut. The bead and arms can be protruded from the shell, and can also be com pletely retracted within it. There are numerous arms attached to the head, 19 in the
best-known species; there arc also numerous other tentacles; but none of these organs tire furnished with suckers, and they are feeble in comparison with the corresponding organs of many of the higher or diliranchiate cephalopods. The mouth is of the par rot's-bill form, as in the other cephalopods; but the mandibles are not entirely composed of horny matter, their extremities being calcareous and of a hardness apparently adapted for breaking shells. Their edges are also notched, and show an adaptation for crushing rather The tongue is large. The gizzard is muscular. The food appears to consist, at least in great part, of crustaceans.
Only three species of nautilus are known, of which the best known and apparently the most abundant is the PEARLY .N.kurrr.us pompilius),which is found in the India' and the Pacific oceans. Its shell is beautifully nacreous within; and is externally por relain-like, white, and streaked with reddish chestnut. The shell, being large, thick, and strong, is used for a variety of purposes by the natives of the East Indies and South Sea islands; it is aDo made into ornaments of various kinds in China and elsewhere. The animal is eaten by the Fijians and other South Sea islanders, and is much esteemed as an article of food. The Fijians capture it by means of a basket-trap, somewhat like those used for catching lobsters, baited with boiled crayfish. The name PAPER NAirrmus has sometimes been given to the argonaut (q.v.).
Fossil .Savtilus.—Abotit ViCI species of fossil shells have been referred to tins genus. They occur in all the strata from the upper silurian to the most recent deposits. Numerous forms, however,which exhibit very wide differences. have been incongruously associated under this generic name. The paleozoie nautili are so remarkable that they must certainly be referred to one or more separate genera; some of the carboniferous spe cies have a square back, and the whorls either compact or open iu the center, while the last chamber is more or less disunited from the shell; and the Devonian clymenia has angular sutures and an internal siphuncle. Until a careful revision of this section of the eephalopoda is made it will be better to consider the species as belonging to the family nautilidce, and not to the genus nautilus.