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Show 18/8.] SHELLS OF CEPHALOPODS. 959 A colleague of Stoliczka, accepting the recorded structure and relative position of Nautilus, frames his nomenclature of the parts and aspects of the fossil shells of the Nautilidee accordingly', the grounds for such being those of positive or direct evidence from dissection. I am led, therefore, to submit remarks on the guiding knowledge in regard to the structure and shell-relations of the extinct Ammonites, which may be gained from negative or inductive testimony. J. E. Gray, we have seen, deemed that testimony to be such in kind and amount as to warrant his rejection of the affinity of Ammonites to Nautilus, and his adoption of the association of Ammonites with Spirula in the Dibranchiate order of Cephalopoda. Examples of unmutilated Ammonite shells are, indeed, very rare; seldom is a specimen acquired with any considerable proportion of the last chamber. Yet some such were to be seen in an accessible museum in London long anterior to 1845. A bisected specimen of the Ammonites obtusus, Sow., in the Hunterian collection (No. 188), shows well the extent of the last or inhabited chamber of the shell, and the effects of the influence of the animal matter of the decaying Cephalopod upon the petrifactive processes after death. The liassic clay has penetrated as far as the retracted soft parts of the Ammonite permitted; the decomposing mollusk has been partially replaced by crystals of spar discoloured by the pigmental or carbonized parts of the animal; the spar which has more slowly infiltrated through the pores of the shell into the air-chambers is of a much lighter colour. In the same collection may be seen exemplifications of injury and repair of tbe cell. In No. 195, Ammonites goliathus, D'Orb., from the Oxford Clay, a portion of the shell, at the period when it formed the dwelling-chamber, "had been broken away during the lifetime of the animal, and has been repaired by fresh material, wanting the ribbed structure of the originally formed shell" 2. The reparation closely resembles that which recent Nautilus-shells occasionally present, and which we know was effected by the formative border of the mantle reflected over the last chamber and applied to the fractured part. No such process could take place in Spirula, the mantle of which is muscular, and inapplicable to the last chamber. There is no need of a living Ammonite to assure us that its mantle, like its porcellauo-nacreous shell, had the same structure as that of the living Nautilus. 1 Memoirs of the Geological Survey of India, 4to, 1866. "The Fossil Cephalopoda of the Cretaceous Rocks of Southern India: Belemnitidce-Nautilidee," by H E N R Y F. B L A N F O R D . " Throughout the following descriptions I have employed the terms ventral and dorsal strictly with reference to the position of the animal, and therefore in an opposite sense to that in which they were used by paleontologists before the anatomy of the animal was known," p 7 (And Mr. Blanford might have added " long after."-R. O.) 2 Catalogue of Fossil Invertebrata, Mus. College of Surgeons, London, 4to, 1856 p. 43, in which work I described upwards of 350 specimens, illustrative of the different families and genera of Ammonitidee, collected by J O H N H U N T ER in the last century. |
