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 Nautilidce accordingly 1 , 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 testi- 

 mony. 



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 Ammo- 

 nites 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 18-15. 



A bisected specimen of the Ammonites ootusus, 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 the cell. In No. 195, Ammonites yoliathus, 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 forma- 

 tive 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 porcellano-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 : Belemnitida — Nauti- 

 lidce," by Henry F. Blanfokd. ''Throughout the following descriptions I 

 have employed the terms ventral and dorsal strictly with reference to the posi- 

 tion of the animal, and therefore in an opposite sense to that in which they 

 were vised by palaeontologists before the anatomy of the animal was known," 

 p. 7. (And Mr. Blanford might have added " long after." — K. 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 Ammo?iitidcp, collected by John Hunter 

 in the last century. 



