Transactions of the Geological Society, 1st series, vol. 2/On Hippurites from Sicily

From Wikisource
Jump to navigation Jump to search

X. Observations on the Specimens of Hippurites from Sicily, presented to the Geological Society by the Hon. Henry Grey Bennet.

By James Parkinson, Mem. Geol. Soc.

THE examination of the excellent series of Hippurites, from Cape Passaro, in Sicily, presented to the Society by the Hon. H. G. Bennet, has occasioned the present endeavour to add to the knowledge of the nature and œconomy of the animal, whose remains are thus beautifully preserved. The specimens are indeed such as demand, that they should obtain the particular attention of the Society, since they possess those characters which will probably serve to correct some erroneous opinions respecting the nature and habits of the animals of which these shells were the dwellings.

These fossils exactly agree with the description of M. Lamarck, being straight or bowed conical shells, furnished internally with transverse septa, and with two longitudinal, lateral, obtuse and converging ridges. These characters are distinctly observable in one of the specimens[1] which contains, with several other remains of these fossils, a nearly perfect shell, longitudinally divided, so as to display very beautifully the two ridges, with the numerous septa and chambers. In another,[2] a longitudinal section of one of these fossil shells, the same parts are shewn, and the extreme thinness of the septa, and the closeness with which they are placed to each other, are particularly observable.

M. Lamarck concludes his description of this fossil by saying, that the last chamber is closed by an operculum; a circumstance which M. Picot de la Peyrouse had supposed that he had frequently observed, whilst examining the numerous specimens which he had found on the Pyrenees. The existence of this operculum is however positively denied by M. Denys de Montfort, who considers this supposed operculum as only one of the septa bearing the impression of the posterior part of the animal. Two specimens[3] contain sections of these fossils, towards their superior termination; and shew, perhaps, the inner side of this last septum or supposed operculum; but do not appear to yield any marks which may assist in determining on the real nature and office of these parts.

M. Picot de la Peyrouse, besides finding several of these bodies of a conical form, whose length did not exceed their diameter, found others of a cylindrical form, some of which were a foot and a half in length, though not more than an inch in diameter; and of course nearly as large at one end as at the other. The regular manner in which he found these fossils arranged by the side of each other led him to describe them as resembling the pipes of an organ. M. Denys de Montfort has thought proper to place the fossils of this description under a new and distinct genus, which he names Batolites organisans, (Le batolite tuyau d'Orge.) The hippurite he remarks is constantly bowed and short, and possesses a very thick shell; whilst the batolite on the contrary is straight and long, and has a very thin shell.

In another specimen[4] a section of one of these fossils is seen possessing a much greater degree of curvature than that which is seen in No. 1. which indeed departs but little from the straight line. These two fossils may be regarded as intermediate in their length and form, between those which have been placed by M. Denys de Montfort in the two different genera, Hippurites, and Batolites organisans; and if this connection be admitted, it may then appear that those differences which that assiduous naturalist has assumed as the foundation of generic distinction, should be considered as at most only marking the distinction of species.

No attempt has hitherto been made to explain the uses of the several parts of this fossil, or to infer from them any thing respecting the habits of the animal itself. M. Picot de la Peyrouse gives the following observations on the subject: “Nous ne connoissons que tres imparfaitement l'analogue marin des Orthocératites. Ceux que Plancus a découverts dans les sables de Rimini, et que Gualtieri a fait graver, grossis aux microscope, suffisent pour convaincre les plus incrédules, que les orthocératites sont de véritables corps marins pétrifiés: mais non pour nous faire connoitre l'usage du siphon, de la gouttiére, des concamérations, &c. Il faudroit, pour y parvenir, découvrir les analogues marins des principales especes de grands Orthocératites, avoir étudié leurs mœurs, leurs habitudes, leur maniére d'être. Que des difficultés désespérantes ne présente pas ce projet! dût il même être rempli, serions nous, pour cela, mieux instruits de 1'usage de cette partie essentielle de l'organisation de cette animal? Le nautile chambré est un des co quill ages les plus cennus. Nous avons la description, on la voyoit en troupes sur la surface des mers, voguer jusques dans les ports les plus frequentés (voyez description du cap. de B. E. tome III. p. 154); nous savons qu'il a des cloisons, et un siphon; nous n'en ignorons pas moins l'usage qu'il en fait? Je crois donc ne mériter aucune blâme, si je ne hazarde point de conjectures sur l'organisation des différentes parties de ces orthocérarites; parcequ'il me seroit impossible de les étayer d'aucune preuve satisfaisante.” (Description de plusieurs nouvelles espéces d'orthocératites, &c. p. 15.)

A due attention however to their peculiar organization would, it may be supposed, have prevented the multilocular shells from having been considered as pelagian shells. But M. Denys de Montfort, with most of the French oryctologists, is disposed to believe that all those shells which are known to us only in a fossil state, and which have therefore been supposed to belong to animals which are now extinct, do actually belong to animals which constantly inhabit the bottom of the sea, never rising to the surface, or appearing on the shores. Thus the orthoceratites, ammonites, and belemnites, notwithstanding their possessing different modifications of that organisation by which the nautilus is enabled to raise itself to the surface of the water, are all considered as petrified pelagian shells, whose recent analogues have not yet been brought to view. Thus speaking of the simplégade couleuvrée (simplagades colubratus), a fossil shell hitherto considered as an ammonite, but which this author has thought right to place under a distinct genus, he says, “ Il est très probable que les simplégades, comme beaucoup d'autres mollusques pélagiens, vivent dans le fond des hauts mers, et qu'une cause physique quelconque ne leur permet point de paroitre à la surface des eaux.” (Conchologie Systematique, &c. p. 84.)

Having already opposed this opinion in a general way, in my Examination of the organic remains of a former world; and having shown that in all the known multilocular fossil shells such an organization existed as was well calculated to enable the animals to raise themselves occasionally to the surface of the water, I shall here endeavour to determine how far the specimens presented to the Society will show, that this property was possessed by the hippurites.

On examining those parts of this fossil, which have been hitherto spoken only of as lateral ridges passing through the chambers and septa, they will be found in most specimens to be formed of a solid spathose substance. But in the specimen No. VI, in which is a transverse section of one of these shells near to its upper extremity, these ridges will be seen, in consequence of the cavities not having been filled by the infiltration of calcareous matter, to have originally possessed a singular organization: a substance of extremely loose and light texture is disposed in plates, which radiate towards the sides of the containing tubes, leaving interstices between them, nearly equal in size to that of the radiating substance itself.

It is indeed impossible to speak decidedly as to the manner in which this peculiar organization could influence the buoyancy of the animal and its dwelling. It is however not difficult to conceive that the animal might possess the power of contracting and of expanding this radiating substance; and might, by thus admitting or expelling the water, produce that change in the specific gravity of the whole mass as might occasion it to sink or rise according as the necessities of the animal might direct.

The great substance of this shell, it being generally about half an inch in thickness, does not, it must be acknowledged, appear at first sight to be favourable to the opinion of its having been capable of being rendered buoyant. But the appearances offered to our observation by the specimen No. VII. give reason for supposing that the texture of the shell was such, that its size might rather promote than prevent its buoyancy. In this specimen the looseness of the texture of the shell is very remarkable; it must however be admitted, that it is difficult to ascertain how much of this depends on the original lightness of structure, the interstices not having been filled up by calcareous matter, and how much on subsequent decomposition.

  1. No. 1. of the Series.
  2. No 2
  3. No.2 & 4.
  4. No. 5.