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Show 962 PROF. OWEN ON THE [Nov. 19, In the application of the anatomy of the constructor of the Pearly Nautilus to the solution of the problem of the nature and function of the Trigonellites, I was led to regard them as the homologue of the organ, or a portion of the organ, in Nautilus, which is " of a fibrous texture, resembling dense corium," called from its shape and position the "hood" (Plate L X . fig. 3, e), and which, when the animal had withdrawn into its dwelling, "would serve as a rigid defence at the outlet of the shell" **. It needed only that this part should be more or less calcified to form the preserved portions of an operculum like that ascribed to the Ammonite. If, for example, calcification had commenced in each half of the symmetrical " hood," and had stopped at the mid line (where the hood is thinnest), the pair of the there-often-suturally-joined symmetrical pieces of the Aptychus would have resulted. The relative size oi Aptychus agrees with that of the shell. It has been found to measure 7 inches 6 lines in length and 6 inches in breadth in gigantic Ammonites2. It may be doubted whether the nidamental glands ever increased in the same ratio ; and it is still less likely that they needed such defensive plates in their season of rest and attenuation. If, therefore, m y homology of the symmetrical halves of the Nautilus' hood with the parial Trigonellites (Aptychus, v. M.) be preferably accepted, the supposition that these parts are calcifications of an Ammonite's hood may be deemed reasonable. That the fibrous basis of the hood was retained in different degrees in the Ammonites is indicated by the simply corneous or chitinous condition of the Aptychus which has been preserved in some examples of Ammonites falcifer, Sow., and its allies. In other species, as in the Ammonites Ungulatus (Plate L X . fig. 1, o), the lateral calcifications have partially met and joined at the mid line; in a third series these opercular plates are thicker and are there suturally united. This is the case in the small or young specimen of Ammonites sub-radiatus, Sow., in the British Museum, which is described and figured by S. P. Woodward, F.G.S., in * Tbe Geologist'3. As the view of the specimen there given is an oblique side one, I here appeud a direct view of the aperture of the dwelling-chamber as closed and protected by its operculum (Plate L X . fig. 2, o). The correspondence of general shape with that of the " hood" of the Nautilus pompilius, as figured in plate iii. fig. 1 of m y * Memoir,' is close4. The conjoined plates of the Aptychus (ib. fig. 2, o, o) form a triangular disk, of which the base is backward, excavated to receive the involute part of the shell, p, with the sides of the base, like the corresponding lobes of the " hood," bent down to cover the laterally extended parts of the wider terminal coil of the shell, q. Even in the contrast between the papillose wrinkled outer surface and the 1 Memoir on the Pearly Nautilus, 1832, p. 12, pl. i. n, and pl. iii. fig. 1. 2 See de Zigno, Memorie del R. Istituto Veneto, torn. xv. tab. viii., 1870. 3 8vo, 1860, p. 328. 4 Woodward gives the following description of its ammonitic homologue:- " The operculum is flat in the middle-, with a slight furrow along the suture, and is much bent down at the hinder corners, where it abuts against the inner whorl of the sheD. It is sculptured externally with about twelve angular concentric furrows; the inner surface is smooth."-B>. ib. |