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Show 1878.] SHELLS OF CEPHALOPODS. 9(31 correspondence of the relative position of the Aptychusl, in the body-chamber, to that of the nidamental glands of Nautilus pompilius2 to the shell; and he deems each valve oi Aptychus to have been applied to a lateral lobe of the nidamental gland3. But these lateral lobes are divided from each other by a part of the middle lobe4, while the valves of the Aptychus are usually in contact (as in Plate L X . fig. 1, o), or may be suturally united along the mid line (as in ib. fig. 2, o). But to return to Waagen's argument from position in the fossil shell. In the course of decomposition after death the calcareous plates would be likely to gravitate or sink deeper into the body-chamber than their natural position in the living Ammonite. So sinking, they would rather lodge or settle in the hollow of the outer (ventral) wall of the body-chamber ( B ) than upon the involute convexity on the opposite (dorsal) wall ( A ) . Moreover there are examples (as in Plate L X . fig. 2) in which the Trigonellites (o, o) have been found in an opercular relation at the mouth of the shell. And, considering the movements to which an Ammonite must have been subject from the time of its death to the solution of the soft parts and final imbedding of the shell in the matrix or seat of its petrifaction, one is prepared for the rarity of the conservation with the shell of its loose operculum, and for the still more rare retention of the Aptychus in its original position. Waagen admits that there are five specimens of Ammonite in the Munich collection exhibiting this position5. He, however, contends that the breadth of the aperture of the body-chamber is less, in certain Ammonites (A. steraspis, e. g.), than the united breadth of the aptychal plates. But so, likewise, if the side lobes of the hood of Nautilus6 were outstretched horizontally, it would exceed the breadth of the outlet of the dwelling-chamber in N. pompilius. But the side lobes of the hood are bent back obliquely in order to close the dorsal side-curved borders, or notches, of that part of the shell-aperture, just as the aptychal lobes or valves are bent down in Plate L X . fig. 2, o, o. It may be further remarked, in respect to the nidamental glands, that they are subject to seasonal changes, and gain the relative bulk with which the size of the aptychal plates accord only at the period of discharge of the impregnated ova, for which they have to furnish the protective coat or nidus. Such seasonal change is exemplified in the figure of these glands given in the ' Memoir on the Nautilus' of 1832, and in that which is shown in Taf. xix. of Waagen's Treatise, 1871. Moreover, in not one of the existing genera or species of Cephalopod, Nautilus included, in which these glands are superadded to the more essential organs of generation, are they encumbered in any way or degree with such calcareous plates as Kefer-stein's hypothesis applies to them in the Ammonite. 1 Paheontographia, 1871, taf. xl. fig. 4. 2 See Memoir on the Nautilus, 1832, pl. i., e. 3 Ib. pl. viii. fig. 10, b, b. 4 lb. ib. a. 5 " Unser Museum besitzt gegen 100 exemplare von Ammoniten mit erhal-tenen Aptychus; unter dieser ganzen Anzahl sind 5, welche den Aptychus in senkrechter Stellung a m Ende der Wohnkammer liegend haben " (op. cit. p. 192). 6 Memoir on the Nautilus, 1832, pl. iii. fig. 1, g, g. |