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Show 1878.] PROF. OWEN ON THE SHELLS OF CEPHALOPODS. 955 6. On the Relative Positions to their Constructors of Chambered Shells of Cephalopods. By Prof. OWEN,C.B., F.R.S., F.Z.S., &c. [Eeceived October 4, 1878.] (Plate LX.) The relations of the chambered and siphonated shells to their constructors can now, owing to the extent to which they have been subject to extinction, only be fully elucidated by a study of species or varieties of two of the genera, Nautilus and Spirula. The fossil shells of Cephalopods, as is well known, exhibit a progressive uncoiling from Nautilus to Orthoceras and from Ammonites to Baculiies, with various modifications of the process ; but that this had been carried out to a coiling in the reverse direction, required anatomical evidence for its demonstration. The insight, however, gained into the organization of Spirula peronii at the date of the publication of the ' Zoology of the Voyage of the Samarang'1, added to that previously obtained on the organization of Nautilus pompilius2, led me to express a conviction of their shell-relations in the following terms :- " These shells (Nautilus and Ammonites) are revolutely spiral or coiled over the back of the animal, not involute like the Spirula "3. And, if the direction of the coils be determined by their relation to the back and belly of the framer of tbe shell, no other interpretation can be given of such relation as it is exhibited in Spirula (Plate L X . fig. 4) and Nautilus (ib. fig. 3) respectively. It is, however, to the exposition and characters of the extraordinary number and manifold variety of the extinct Cephalopods, now known only by their polythalamous and siphonated shells, that an exact and accepted determination of their relative position to the body of the framer is most needed. In the year 1829 L E O P O L D V O N B U C H initiated, in his notable memoirs on tbe Ammonites4, the definition and nomenclature of tbe shell-characters by which the species, genera, and families might be defined. Assuming, and correctly in my judgment, that the shells of an Ammonite and a Nautilus were coiled in the same direction, he premises : - " Le caractere distinctif entre ces deux genres de Cepha-lopodes, consiste en ce que le syphon des Ammonites est toujours dorsal, et qui'il ne l'est jamais dans les Nautiles "5. Next, calling attention to the lobed and foliaceous sutures of the Ammonitic shells, he defines the parts which he calls "lobes" and "selles" (saddles)- specifying of the former, " le lobe dorsal," " le lobe ventral," and " les lobes lateraux"*. Non Buch's descriptions and figures admit of no 1 "Mollusca," Part 1, 4to, p. 6, pl. iv. 2 Memoir on the Pearly Nautilus, &c. 4to, 1832. 3 Palaeontology, 8vo, 1861, ]>. 97. 4 Annales des Sciences Naturellcs, 8vo, tome xvii. p. 267, tome xviii. p. 417. 6 Ib. p. 268. 6 lb- ib. |