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OBSERVATIONS ON THE ANIMALS INHABITING MULTILOCULAR SHELLS,

CHIEFLY WITH A VIEW TO THE GEOLOGICAL IMPORTANCE OF THE SUBJECT.

By D. T. ANSTED, ESQ. B.A. F.G.S. F.C.P.S.

OF JESUS COLLEGE, CAMBRIDGE.*

IN a former paper on this subject, which ended rather abruptly, in consequence of a page of MS. having been mislaid, I was proceeding, after certain general remarks, and some account of the family Nautilacea and two genera of Ammoneata, to describe the sub-genera of the important and widely-extended Ammonites; and it may be remembered that the first of these, the Goniatites, was the only one of which any extended account was given. Next in order, the Ceratites of de Haan were mentioned as occurring in the muschelkalk beds of the continent; and these differ chiefly from the Goniatites in the nature of the intersection of the chamber with the shell, which in the latter group is angular, whilst in the former it consists of a series of nearly regular curves, alternately semicircles and mere wavy lines. But there is a further difference, causing a nearer approximation to the true Ammonites, in this group; for, instead of the shells being very round and smooth, as the Goniatites almost invariably are, they begin to be provided with tubercles, acting as a sort of compensation for a flattening of the spiral, which generally occurs when these are introduced: for this flattened form, not being so strong with the same thickness of shell as the more rounded shape and more perfect arch of the other group, the tubercles or bosses are introduced, "superadding the strength of a dome to that of the simple arch at each point where these bosses are inserted."

Passing on now to the other Ammonites, we find the line of intersection already spoken of waved in a more and more complicated manner, till it resembles rather the edge of a parsley leaf, or the sutures of a skull, than any thing else one can compare it to, and at the same time the number of bosses or tubercles is increased, and additional contrivances for strength are introduced, the shell often becoming extremely fragile and the siphuncle very minute. chalk Ammonites especially the shell is very thin; and it is often a

* Continued from page 50 of the last number.

matter of some difficulty, even in large specimens, to discover whereabouts the aperture for the siphuncle is situated.

Now, with regard to the use of this extreme complication of the edge of the chamber, although it certainly seems to add much strength to the whole, there is a secondary object mentioned by M. von Buch, which we must not neglect to notice. He considers that the alternate projections and recesses thus formed must have given firmer hold to the mantle of the animal, and enabled it to retain, in safe and close connexion, the animal and its shell, notwithstanding the small size and inconvenient position of the siphuncle, which, in all probability, is of much use for this purpose in the Nautilacea. Following out the idea, he has traced a remarkable uniformity in the number and positions of the undulations throughout the whole genus of Ammonites. "I think," he says, "it may be considered a question definitively settled that in all species, whatever may be the apparent anomalies of form, it is easy to make out six principal lobes,* with other accessory lobes interposed, which all adjust themselves with wonderful regularity in the circumference of the shell." Between every pair of depressions or lobes there is a raised rounded part, the saddle; and these saddles always correspond in number and position to the lobes.

The system of lobes and saddles, so constant and disposed with such exact symmetry, appeared to M. von Buch to indicate an orga nization separating very decisively the two families Nautilacea and Ammoneata; and although, perhaps, the conclusions drawn from this view of the subject, and the sub-divisions proposed, are premature, and the facts already determined not quite so important as they have been assumed to be, still much good has been done by the mere convenient classification of so large a genus into distinct groups, which, indeed, our author goes so far as to call "une distribution claire, positive et utile, en familles naturelles."

We have one more remark to make on the Ammonites generally, before proceeding to the consideration of the allied genera. Their shells, it would seem, are usually thin, and the siphuncle small; but the former receive great additional strength by the number of ribs, as well as tubercles, not immediately connected with the septa, and fluting, as it were, the part most exposed to pressure; while the

By the word "lobe" is designated the depressions which occur in the wall of the chamber of Ammonites, &c. caused by the successive bends of the undulations towards the aperture. The raised parts between these hollows are called saddles.

latter the siphuncle-is often undefended, being almost always placed at the extreme dorsal edge of the shell, and sometimes actually outside it.

Lastly, with regard to the distribution of the remaining groups of this genus, it is important to observe that the period throughout the formation of the oolites was by far the most abundant, in every way, in species, as well as individuals. Even in the limited state of our knowledge of the animal kingdom at that time, we can speak to not less than a hundred and fifty distinct species, every one created, living its appointed time, and becoming extinct, between the commencement and close of that series of limestone deposits.

In all parts of the continent of Europe, as well as in Great Britain, the fossils of this genus are extraordinarily abundant ; but they are not confined to Europe. Specimens have been found very high up on the Himalaya mountains, in Asia. They occur in the state of New Jersey, and in several other places in North America; in Brazil, and on the coast of Chili in South America: and probably, when future researches shall have laid open the scientific stores of Africa and Australia, other species will there be found, showing the genus to have been once as widely distributed as it was undoubtedly locally abundant.

It is rather singular that out of a number of allied genera, forming together an important natural family, one genus should be among the most widely spread of any that is known, and all the others comparatively very rare, and occupying no important place in the scale of nature. Yet so it is in the case before us; for the Ammonites are not more remarkable for their singular variety and great numbers, than the several genera, closely allied in every thing but external form, are for the very narrow limits within which they are confined. The Hamites, next in order to the Ammonites, are not, indeed, quite so rare as the Scaphites or Turrilites already described; but still they are only met with in a few strata, and in but one of those are they at all abundant. Of the different species known, two occur in the continental beds of the oolites; one so low down as the lias, while there are nearly thirty in the cretaceous group, most of them occurring in the two beds of green sand.

Of the Baculites, which come next, five species only are named at present, and all appear to have lived during the deposition of the chalk. The difference between these two last genera consists, to all appearance, in a very unimportant change of form, the Hamite being, as we have already observed, bent round more or less at the smaller extremity or apex of the long, narrow, and often elliptical

cone, which is the simple and ordinary form of the Baculite. Both are sometimes ribbed, though the latter rarely; and in both the edges of the septa are very complicated. The shells of both, too, are commonly rather thin, the siphuncle not large, and the complete fossil, especially of the Baculite, extremely rare.

So much for the external form of the shells referred to the great natural family Ammoneata; a family which deserves well the consideration of the paleontologist, since of all others it is, as we have already remarked, the most widely spread, the most characteristic of several formations, and which is, moreover, just so nearly allied to living types as to excite our curiosity, and raise our hopes that something may sooner or later appear, some new discovery or closer observation made, by which our speculations may be tested, and their truth or falsity proved.

With a full knowledge, then, of the danger of too rash generalizations, and wishing it to be clearly understood that our theories and attempted analogies are rather thrown out with a view to excite inquiry than with any expectation of satisfying doubts, or still more of deciding dogmatically on any disputed point, we will proceed to the consideration of the following queries, viz. how far the analogies traceable in the shells of the Nautilacea and Ammoneata are indicative of analogous habits, and to what change of habits of the animal the alteration of structure in the two families may most probably refer.

Now, that the subject may be more perfectly understood, let us here just repeat, in as few words as possible, the most important both of the analogies and differences, because, when they are thus put in apposition, our plan of considering the matter will be more clear to the reader, and its value, however small, more truly appreciated.

The shells, then, of the two families, are, in all cases, multilocular, the chambers being filled with air during the life of the animal, and forming then a mass not very different in weight from the weight of an equal mass of water. In other words, the shell would float of itself under the ordinary conditions. Besides this, the walls of separation of the chambers are always pierced with a hole larger or smaller, through which a tube has passed, communicating with the interior of the animal. There is, in all the species, considerable regard paid to the general strength of the shell in some way or other, more especially where the animal is no longer present, for the empty cells are stronger than the outer one, which is, in most the only one inhabited.

Such are the analogies: the differences are sufficiently remarkable, and refer chiefly to the walls of separation or septa between the chambers, the siphuncle or tube running through the chambers, and the peculiar methods adopted to strengthen the resisting power of the empty cells.

The first difference is that relating to the septa. In the Nautilacea these are simple plates, concave outwardly, and presenting a nearly regular curve in their intersection with the shell. On the other hand, they are in the Ammoneata, always more or less circular in their form, and the line of intersection varies from a kind of zigzag, to as complicated a curve as can be imagined to exist. There is, thus, in the species of this family, a contrivance by which each internal transverse plate, presents a much longer line for the shell to press against, or, if you will, offers a much greater number of points of resistance than in the more simple-formed Nautilacea.

The next point of difference to be attended to, is in the siphuncle, and it is one both of position and magnitude. In the genera of the first family, this important part is sometimes ventral, or on the inner margin, more frequently central, and is very rarely observed to approach the dorsal or outer margin. On the other hand, it is almost always very near the dorsal margin in the Ammoneata, and sometimes is actually placed outside, in a channel opened for it, and projecting from the back of the shell in the shape of a keel. The difference in size is not so constant; but it is not, certainly, too much to assert, as a general rule, that it bears a very much smaller proportion to the area of a section across the chamber in Ammonites and their congeners, than in the Nautiloid forms. The siphuncle, then, is, on the whole, more undefended, and apparently less important, in the former than in the latter.

Lastly, there is the difference of plan resorted to in the two families to accomplish the same purpose, that of giving a degree of strength not found in other shells. In Nautilus, and generally in the Nautilacea, the most simple means are employed. The shell is tolerably thick, the whorls often successively envelope each other, and the surface exposed forms a tolerably strong single arch. Not so is the case with most of the Ammoneata. The shell is usually extremely thin, even in the very large species; the whorls do not, or at least very rarely, envelope; and the spiral disc, being flattened, as it often is in the transverse section, is frequently elliptical, and sometimes almost angular. But now for the compensation. Instead of a simple arch, we have one which is fluted; and we find a set of contrivances, consisting of ribs and domes, with transverse