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Show 9G0 PROF. OWEN ON THE [Nov. 19, The perfect specimen (Plate L X . fig. 1) of the last chamber of the Ammonite was derived from that eminently conservative matrix, the lithographic slate of Solenhofen, Bavaria. The extent of the outer curve of the dwelling-chamber to the outer border of its floor (the last septum, x to y) is 4 inches; the extent of the outer curve of the chambered part of the shell to the inner border of the chamber- floor (last septum, y to z) is 6 inches. In the Nautilus pompilius transmitted in its perfect shell to Prof. Vrolik the corresponding admeasurements are 7 inches 9 lines and 11 inches 6 lines. The correspondence in the proportion of the dwelling-chamber to the camerated part of the shell in the Ammonite and Nautilus is thus, as in numerous other instances, shown to be instructively close. In the last chamber of Ammonites linyulatus, moreover, as if to proclaim to the most sceptical its function of lodging its constructor, is preserved the only fossilizable part of such (ib. fig. 1, o). Since the publication of the ' Paleontologie' of Pictet1, who refers the Trigonellites to the Cirripeds, abundant evidence has been obtained of the accuracy of the opinion of Volz2, that the Trigonellites of Parkinson3 (Aptychus of v. Meyer4) were parts of the animal of the Ammonite, and stood in opercular relation to the shell of that extinct Tetrabranchiate. Pictet takes no note of the confirmation contributed by Morris to the Vblzian examples of Trigonellites within the last or dwelling-chamber of Ammonites, in the portion of an Ammonites walcotti in which "the Aptychus, oi a corneo-calcareous nature, was found imbedded in the matrix filling the last chamber about 6 inches from the aperture " \ This common, though not constant, position led some palaeontologists to surmise that the Trigonellites might be parts of the Ammonites' gizzard, like the triturating plates in Bulla lignaria.. M . Valenciennes believed them to be lateral supports of the funnel6. Van der Hoeven "hazards the opinion that the two juxtaposed fossil shells, known by palatographs as Aptychus, were two shelly supports of the hood of Ammonites " (Trans. Zool. Soc. vol. iv. p. 22). In this opinion I concur; but it has been rejected by some experienced students of the extinct polythalamous shells. Keferstein and Waagen7, e. g., deem the Trigonellites to be sexual characters, and to have served as protective plates of the nidamental glands of the female Ammonite. Waagen adduces, in support of this view, the 1 Traite" de Paleontologie, &c, vol. ii. p. 551, 8vo (1854). 2 "Sur le Belopeltis," in Memoires de la Soc. d'Hist. Nat. de Strasbourg, tome iii. (1836). 3 'Organic Remains of a Former World,' 4to, 1811, p. 184, pl. xiii. figs. 9-12. * Nova Acta Acad. Nat. Cur. t. xv. pt, 2, p. 125, pl. lviii. (1831). 8 " Note on Aptychus," in Ann. & Mag. Nat. Hist. vol. x. (2nd ser.), p. 356, pl. v. D. fig. 1. 6 " Je crois qu'il faut admettre que l'entonnoir de l'ammonite, s'il rStait forme de deux valves, ne contenait pas de cartilage interne; mais que cette piece etait reinplac6e par un organe exterieur compose de deux pieces paires syinetriques comme le sont les aptychus."-Ib. p. 304. 7 "Ueber die Ansatzstelle der Haftmuskehi beim Nautilus und Aminoniten," Palseontographia, 4to, p. 185, taf. xxxix., 1871. |