|On submicroscopic structure of the Nautilus shell|
Grégoire, Ch. (1962). On submicroscopic structure of the Nautilus shell. Bull. K. Belg. Inst. Nat. Wet. 38(49): 1-71
In: Bulletin. Institut Royal des Sciences Naturelles de Belgique. Mededelingen. Koninklijk Belgisch Instituut voor Natuurwetenschappen. KBIN: Brussel. ISSN 0368-0177, more
1. The submicroscopic structure of the organic and mineral components of dried shells of Nautilus pompilius LAMARCK and Nautilus macromphalus SOWERBY bas been investigated with the electron microscope.2. Approximately 350 different areas of the shell wall and of the septa, in the living chamber and in the phragmocone. have been studied on positive metallic replicas of original surfaces, of surfaces of fracture, of polished transverse and longitudinal sections, before and after etching with chelating agents. The structure of the organic material has been studied on decalcified fragments of the different regions, thinned by teasing or by disintegration by ultrasonic waves. 3. Various forms of crystals and of crystal aggregates were detected on the outer and inner surfaces of the shell wall and of the camerae. In the crystals lying on the nacreous growth surfaces of the living chamber the tabular hexagonal form (001) (010) (110) of the truncated orthorhombic prism of aragonite is predominant. The c-axis of these crystals is normal to the nacreous stratification. their a-axis, along which many crystals are elongated is predominantly at right angle to the growth lines. Parallel orientation and aggregation. parallel overgrowth of micro-crystals oriented along the a-axis of the basal crystals were recorded in groups of neighbouring crystals but orientation differed in other adjacent groups. Orientation of the crystals with their a-axis parallel to the direction of the trabeculae of the reticulated sheets of conchiolin on which these crystals had developed by epitaxial growth. was observed in some preparations, but was not obvious in others. The present conclusions about the structures of the weathered surfaces will be amended when fresh shells will be available. 4. The membraneous disc or sole interposed in the living chamber between the muscles and the shell wall. is composed of stratified. cleavable layers of thin microfibrils arranged in dense feltings. Near the adoral and adapical ridges of the muscle scars. globular or lenticular microcrystals clustered in considerable numbers are scattered over the surface of this membranous disc and over that of the underlying shell wall, composed of a variety of mother-of-pearl ( helle Schicht). The number of these microcrystals, estimated in several fields, varied between 2 and 126 millions per square millimeter of surface. 5. The surfaces of the concave and of the convex sides of the septa differ greatly in their structure. On the concave side, large, mostly hexagonal tabular crystals ( 001 ) ( 010 ) ( 110 ) , elongated along their a-axis, are lying on the youngest nacreous lamellae and partly embedded in conchiolin sheets. On the convex side, conical or pyramidal structures, composed of stacked discs, are scattered among shreds of the posterior membrane. 6. The posterior membrane which coats the convex side of the septa is composed of microfibrillar layers. 7. The basic mineral structure representative of the porcellaneous substance in the outer layer of the shell wall and in the umbilical callus, consists of mostly elongated, sharp-edged tablets, blades, plates or bars, appearing in transverse or oblique sections, in the form of girders, beams, rods, needles or spindles. These elements are generally assembled in bundles, plates, rows or corpuscles. The composition of these groups, which varies in the different porcellaneous regions, is described. At the border line of porcellaneous and nacreous substances in the shell wall, the elongated elements are predominantly parallel to the c-axis of the nacreous crystals, and normal to the nacreous stratification. An identical disposition was detected along the fringes of porcellaneous substance deeply notched in the neighbouring nacreous substance, in the region of the umbilical callus and in the sutural substances, The organic sheaths interposed between the elongated porcellaneous elements of the outer layer of the shell wall and of the umbilical callus are composed of microfibrils associated with veils. 8. Particularities of structure characterizing the nacreous layers of the shell wall and those of the septa are described. Periodic disturbances in the stratification of the nacreous lamellae consist of variations in the growth along their c-axis and in the orientation of the aragonite crystals involved in the structure of the lamellae. Differences in the crystal shape, size and disposition between the early adapical and ultimate adoral septa reflect differences in the rate of crystal growth of the respective formations. These findings suggest the possibility of extending the physiological and morphological studies of the periodicities in the shell growth to a submicroscopic level. 9. In Nautilus pompilius, the pattern of the lace-like reticulated sheets of nacreous conchiolin of the shell wall is closely similar or identical to that detected previously in Nautilus macromphalus. This pattem of structure of nacreous conchiolin ( « nautiloid pattem » ) found in two of the three surviving species of the living genus Nautilus appears to be a reliable taxonomic character. The septal nacreous conchiolin differs by its tighter texture and by its more slender trabeculae from that of the shell wall. 10. In the areas of junction of the septa and of the inner surface of the wall, the mural and septal nacreous substances are not contiguous. Layers of calcareous substance, with a structure thoroughly different from mother-of-pearl, are interposed in between. These substances, or sutural infillings, include: 1 ) the linear milk-white cements sandwiched between the shell wall and the mural parts of the septa, 2) the extensions of this material which clog the angles of intersection of the wall and of the convex sides of the septa, 3) the calcareous concretions which overlap the angular infillings and protrude into the cameral cavities. These substances, especially the cements, are composed of layers made up of parallel imbricated, elongated crystals, .disposed transversely in the layers, The layers alternate with strands of organic amorphous substance. The elongated crystals are identical to those which characterize the porcellaneous substance. The organic material interposed between these crystals is similarly composed of microfibrils and veils. The calcareous concretions protruding into the cameral cavities are characterized by a disorderly amalgamation of bundles of crystals and of densely stratified sheets of amorphous mineral substance, in which substantial organic matter is embedded. 11. The findings collected with the electron microscope are discussed with reference to particularities of the test configuration in fossil Nautiloidea reported in literature. A comparative study of the sutural infillings of the recent Nautilus shell and .of the cameral deposits of Pseudorthoceras knoxense has revealed close similarities in the structure of these two kinds of sub~ stances. It is assumed that the wedge-shaped plug of calcareous substance which fills the angles of intersection of the shell wall and of the convex sides of the septa in the recent Nautilus might be the morphological equivalent. confined to a restricted area, of the cameral deposits of fossil Nautiloidea.