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, o) 

 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. pi. hi. fig. 1,/), 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 dwelliug-chamber has been in any propor- 

 tion 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 1 . 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 compen- 

 sated, like that of the 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 im- 

 plying 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 diffe- 

 rence 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 Pictet 2 and 

 M'Coy 3 , 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, Briclgewater Treatise, vol. i., 1835; Owen, "On certain Be- 

 lemnites from the Oxford Clay." Phil. Trans. 1844. 

 - Palaeontology, vol. ii. p. 618. 

 3 Ann. & Mag. Nat. Hist., 2nd ser. vol. viii. p. 481. 



