The multilocular shells are among the most remarkable fossils, on account of their great numbers in that state, and the few that are found living. Nautilus is one of the most remarkable of these, as well for this reason as on ac count of the very obscure economy of the animal when alive. Many fossil species occur in England. The Or thoceratite, a shell with a similar economy, but of a differ ent form, is also interesting as being among the most an cient fossils. To these the Hippurites bears also an ana logy in the concameration and the siphunculus, but it is comparatively rare.
But as the nature of these multilocular shells, and the use of the siphunculus is an interesting subject, equally concerning the whole tribe, a few words on it will not be misplaced. This structure is the more worthy of notice, because it characterizes so large a proportion of the Fossil shells, and is so rare in the living ones. Their numbers, or quantity, are enormous, nor is the number of the spe cies small.
These are distinguished from the other multilocular shells, such as Serpula, Siliquaria, Dentalium, &c. by their having a communication through the septa, by which the cavity of the shell is divided. It is supposed that the animal resides in the last formed chamber, and that it be longs to the cephalapodes, being a creature somewhat re sembling the septa. It is also thought that part of the shell is enveloped by the body of the animal, indepen dently of the connection which it has with the whole of it by means of the siphunculus. It is in this organ, in par ticular, that many genera of this tribe differ. In the nau tilus, that tube is not testaccous, as some have supposed, but membranaceous ; and in this, as in the whole, it is probable, dilatable and compressible, so as to be capable of rendering the creature buoyant or otherwise at pleasure.
as the air-bladder does in fishes. In this genus, also, the membranous tube is extended over the inner surface of the testaceous one, where it is reflected on the exterior, and so on, through all the compartments, even to the extre mity.
By what power the animals so constructed supplied this tube with air, is not known, but the same difficulty exists with regard to the air-bladders of fishes. The cells themselves seem to answer no other purpose than that of containing air, as the animal leaves the last formed one in succession as it constructs a new one, keeping up no other communication with them than that which is pre served by the siphunculus. Thus, as the creature in creases its size, and occupies a new shell or cell, the last in order, and in the same way all the preceding, are left empty, so as to counteract the increasing weight of the shell, and render the whole so nearly equal in weight to water, that a small addition of air by means of the dilata tion of the siphunculus may render it buoyant, or the re verse. In the recent nautilus, the tube always passes
nearly through the middle of the septa, but in some of the fossil ones it passes near the edge.
In the Spirula, the siphunculus is one continuous shel ly tube, the smaller end of each anterior one terminating in the larger end of the nest posterior. It is consequently rigid, and the way, therefore, in which it acts is not well understood.
In the Orthoceratite this tube is exceedingly variable in size, being sometimes not more than a tenth of the dia meter of the shell, and reaching from that up to one half. The shape of it also varies, according to the part of the septa through which it passes ; and it is farther enlarged wherever it passes through the cells, to be contracted again as it perforates the division between these.
In the Belemnite the siphunculus is always upon the margin of the septum, but the general economy of the ani mal appears to have been the same.
The Ammonite seems to have possessed exactly the same power of buoyancy, notwithstanding its occasional great size, as the shell is always thin and light. The form of the siphunculus in this genus has not been made out so accurately, but there is no doubt of its existence, and that the general economy of this animal is the same as that of the nautilus.
The belemnite is a fossil, popularly very well known, as it occurs in many calcareous strata every where. Its true nature was long misunderstood ; and, in consequence, it went by a variety of names, being supposed the spines of an echinus, the horns of some animal, and so forth; among the common people these fossils are often known by the name of thunderbolts. It is now well known to be a cham bered shell, formed on the same general principle as the nautilus. There is, as in that genus, a siphunculus passing through all the cells, and which has, in all these cases, been supposed intended to assist the animal in regulating its buoyancy in the water ; a general counterpoise.existing be tween the total weight of the animal itself and the air con tained in the empty cells, which any slight change in the state of this connecting tube was sufficient to alter, for the purpose of rising or sinking. The cellular part is inclosed in a spai:ry mineral structure, radiating from the axis. It is not certain what this has been in the original animal, but it is supposed to have been a soft cellular or pulpy covering. Beleninites are sometimes found to have been perforated by worms, the passages of which have subsequently been filled by silica, so as to produce very intricate appearances after the calcareous part has been dissected by an acid. This is supposed to be one of the extinct genera, as no thing analogous has yet been found alive. It is common through all the strata from the lowest to the highest in the series, and is also abundant, but the species are not very nu merous.