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Show 1878.] SHELLS OF CEPHALOPODS. 903 smooth inner surface of the "hood" of Nautilus the resemblance to the "Aptychus" of the Ammonite is carried out. Anterior to the apex of the triangular operculum in the Ammonites subradiatus a small portion of the dwelling-chamber (ib. v) is left uncovered, through which a slender stream of water might pass from the retracted funnel. As the soft parts of the Ammonites Ungulatus decayed and were dissolved the calcareous or opercular parts of the hood (ib. fig. 1, 6) have subsided to near the bottom of the dwelling-chamber, probably with such change of their original relative position, as exemplifies the value of the demonstration given in the specimen described by Woodward. If an independent centre of calcification were set up in the mid region of the "hood" (Mem. cit. pl. iii. fig. t,f), a part corresponding to the Anaptychus might result. In further ventilation of the mooted affinity of Ammonites to Spirula, it may be remarked that in not one of the examples of Ammonites in which the dwelling-chamber has been in any proportion preserved has there been any trace of an ink-bag. Yet fossili-zation of this or of its secretion is abundantly exemplified in the extinct Belemnites*. Hence the inference may as confidently be drawn, as from a dissection of the animal of the Ammonite, that this Cephalopod lacked, like the Nautilus, the singular defensive contrivance with which the more active Dibranchiate Cephalopods were endowed, and that the animal of the Ammonite was compensated, like that of tbe Nautilus, by having an external protective shell into which it could retreat and close the entry against the assaults of an enemy. Moreover, admitting the homology of the "Aptychus" with the "hood," we further learn that the defensive door of the house was " dorsal," and that the relative position of the soft parts to the external shell was the same in Ammonites as in Nautilus. It cannot be averred, therefore, in excuse of a nomenclature implying a different and opposite relative position of the soft parts to the shell, that " the animal of the Ammonite is unknown." To the composite porcellano-nacreous structure by which the Ammonitic agree with the Nautiloid series of shells, and their difference in this character from the simply nacreous structure of the Spirula-shell, reference is here made in illustration of the " Law of Correlation." The conformity, in this respect, with the Nautiloid series is maintained under every modification of shape from straight to convolute. But the persistence with which monographers of these numerous and beautiful fossils, notwithstanding the appeals of Pictet2 and M'Coy3, and the practice of Barrande, adhere to the erroneous views of Von Buch as to which was the dorsal and which the ventral aspect of the shells, has moved me to supplement the original grounds of 1 Buckland, Bridgewater Treatise, vol. i., 1835; Owen, "On certain Belemnites from'the Oxford Clay," Phil. Trans. 1844. 2 Palaeontology, vol. ii. p. 618. 3 Ann. & Mag. Nat. Hist., 2nd ser. vol. vm. p. 481. |