A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) ... / With 7 plates and 96 textfigures.

British Museum (Natural History). Department of Geology

Date:: 1911

Credit: A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) ... / With 7 plates and 96 textfigures. Source: Wellcome Collection.

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$Gallery VII. Table-case I. lay the much constricted visceral cone. H.vamination of polished sections across Endoccras and allied forms shows within this septal neck-tube yet another series of structures, called sheaths, and somewhat like a ]>ile of funnels stuck one inside the other (Kigs. 79 c, 80 a). These indicate that, as the animal advanced in its shell, its viscera naturally went with it, and towards the void thus left the walls of the visceral cone were still further sucked in. Thus there tended to ^— a/ierture of shell ■ hody-chamher \ jiartly filed by f • matrix ' ■ shell-ieall sejdum se2>tal neck f siphiiitele suture, seen between matrix filHiig cham- bers ornamented outside of shell-wall apex Fig. 80.—Primitive Nautiloidea. a, diagram of a section through the middle of Piloceras, the dotted lines being reconstructed; sw, shell-wall; s, chambers divided by septa, whose necks form the wall {to) of the wide neck-tube; sc, space occupied by visceral cone, the hardened skin of which forms the sheath (s/t); r, remains of similar sheaths; c, endosiphon. (After Poord). b, Orthoceras; in the lower part the shell-wall is preserved; then it is partly removed, showing the filling of rock or matrix; higher up this is cut through, and at the top it is removed, showing the other side of the shell. arise a narrow and empty tube—tlie sipluuicle. The walls, however, were stiffened with lime and did not completely yield to the suction, so that, when the animal again advanced, the inner layers of the skin were torn away from the outer ones. These inner layers thickened and stiffened in their turn, and the process was repeated. Thus arose the thin tube, sometimes called the endosiphon, and a series of sheaths attached to it. In Orthoceras and similar forms tliero is an advance on$