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C U T T L E-F I S H
Both these modifications seem adapted to give greater strength in relation to a more active life. The modern Decapod Cuttle-fish are undoubtedly the descendants of the Belemnoids. The earliest of these belong to Triassic times where they are known as Aulacoceras, Atractites, and Phragmoteuthis. It is very probable that there were earlier, as yet undiscovered, representatives, and that a cleavage had already taken place, for in the succeeding, Liassic, period a new group is well established. In this group, the Chondrophora, all signs of the original septate shell are lost, and the protecting pen, now horny, alone is left. The genera, known as Tevdopsis, Beloteuthis, Geoteuthis, &c., differ in little beyond their broader pens from the modern Calamaries, so that they rapidly attained their acme of development, and then remained stationary. The other group, the Phragmophora, developed the Bdemnites proper. These retain throughout their whole development the original thin shell—or “ proostracum,” its internal septa and siphuncle forming together the “ phragmocone.” It is the guard that changes, either in form or markings. It is very feeble in Belemnoteuthis and said to be absent in Conoteuthis; it commences to be absorbed in Belemnitella, and is caducous in Actinocamax. So far we are on safe ground, for the Belemnites were certainly Cuttles, since their guard is known to have been internal, by its earlier layers being enveloped by the later ones; and further, they are found with the guard in position at the extremity of a perfectly outlined body with characteristic fins, ten arms with hooks, ink-bag, and eyes.
latter the majority of the later Ammonoids have descended, the genera depending principally upon the build and ornaments, so called, of the shell; till in Cretaceous times their abnormal forms, Hamites, Baculites, &c., indicate their decadence and presage their disappearance. Other abnormal forms have, however, appeared at various epochs of Ammonite history. What became of the Ammonites ? and whence are the Octopods derived ? are two correlated questions on which more light is desirable. The wide distribution of Ammonites in all latitudes and in all varieties of strata, shows that neither change of temperature nor of circumstance can have caused their extinction. The forward convexity of their septa suggests the secretion of gas at great depths which expanded as they rose to the surface before the last septum was fully formed. In this case they must have been pelagic animals, and therefore have been superseded by other pelagic animals performing their function more efficiently—such as more highly developed members of the same class. It is now known that Octopods were actually in existence in the Cretaceous period, thus overlapping the
The history of the divergence from the general Belemnoid type into the two branches represented respectively by the S'pirula, with phragmocone and no mucro (or modified guard), and the Sepia, with macro and no phragmocone (unless the cancellous tissue represent it), is obscure. For the only possible suggestion for an ancestor of the former would be the doubtful Conoteuthis, and the latter is said to have a near ally in Coccoteuthis of the Solenhofen limestone. Both of these are anterior to the disappearance of Belemnites, so that it is not certain whether the latter have been modified into, or superseded by, the representatives of the modern Cuttles. Nor is it clear whether the peculiar cancellous substance in the Sepia represents a series of modified septa, or of special shell layers. Each lamella is supported by sinuous partitions rising vertically from the one below, and these appear to spread out horizontally at intervals and coalesce to form the new lamella, the whole being calcified in horizontal layers. The spaces bounded by the calcified partitions and the lamellae are occupied in the very fresh shell by a number of thin walled, apparently empty, cells in two or more horizontal layers, whose adpressed walls give the appearance of free membranes, but these are without regularity. It is probable that these secrete some gas, so that the lamellae coincide in function, if not in origin, with the septa of a phragmocone. The history of the Ammonoids, including their probable modern representatives, the Octopods, is more complex. In this group the first element to form a centre of variation was the septum, and its outline the suture. This variation in the early stages of growth has been made the basis of Branco’s classification into the Asellati, Latisellati, and Angustisellati 5 in the later or adult stages it gives the principal generic characters of the Goniatite and Ceratite groups, and of those remarkable forms of the Trias in which it reaches its acme of complexity, as in Sageceras, Pinacoceras, and the Arcestidce. After this, the Ammonoids settle down into a “ normal ” type of suture, which, though showing abundant differences of detail, can all be described, without straining, as modifications of a definite arrangement of “ lobes ” and “ saddles.” The centre of variation next passes to the build and ornament of the external shell. Whilst the sutures were the principal seat of variation the shells remained smooth.—-Leiostracanow they became ornamented or rough—Trachyostraca ; doubtless this, on the principle of corrugated iron, made them stronger and less dependent on their septa. From these
Fig. 2.—Palceoctopus Neivloldi, the oldest Octopod known. From the Cretacious rocks of Lebanon. {After H. Woodward.) Ammonoids in time, since a member of that group, the PalcBOdopus (Fig. 2), has left an impression of its body, head, arms, and two fins, as in the Pinnoctopus, on rocks of that age in the Lebanon. This was naked, so that in part of the group the shell was already dispensed with. On the other hand we have in the living Argonaut an Octopod whose shell has a great external resemblance to
